In vivo gene sensors

ABSTRACT

Described are methods and compositions for the detection of target genes. The inventors have developed a synthetic nucleic acid sensor-effector gene circuit. In cells without a target gene, the circuit suppresses e.g., effector production, but in the presence of the target gene the suppression is subject to competition, such that the synthetic sensor is de-repressed and permits expression of the effector gene. The methods and compositions described further permit the selective expression of an effector gene in those cells expressing the target gene. In this manner, cells expressing a target gene can be selectively targeted for treatment or elimination. In certain aspects, the methods and compositions described permit the selective expression of an agent such as a therapeutic gene product, in a specifically targeted population of cells in an organism.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is an International Application filed on Feb. 17, 2009,which claims the benefit under 35 U.S.C. §119(e) of U.S. ProvisionalApplication No. 61/029,158 filed on Feb. 15, 2008, incorporated byreference in its entirety.

GOVERNMENT SUPPORT

This invention was made with Government Support under Contract No.EF-0425719 awarded by the National Science Foundation. The Governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to the detection of cells expressing atarget gene.

BACKGROUND OF THE INVENTION

Bacteria rapidly develop resistance to antibiotic drugs within years oftheir introduction to the clinic¹. Antibiotic resistance can be acquiredby horizontal gene transfer or can result from persistence, in which asmall fraction of cells in a population exhibits a non-inheritedtolerance to antimicrobials². Since antimicrobial drug discovery islagging behind the evolution of antibiotic resistance, there is apressing need for new antibacterial therapies³.

Bacterial infections are responsible for significant morbidity andmortality in clinical settings³. Though the advent of antibiotics hasreduced the impact of bacterial diseases on human health, the constantevolution of antibiotic resistance poses a serious challenge to theusefulness of currently available antibiotic drugs³⁻⁷. Infections thatwould have been easily cured by antibiotics in the past are now able tosurvive to a greater extent, resulting in sicker patients and longerhospitalizations^(5, 8, 9). The economic impact of antibiotic-resistantinfections is estimated to be between US $5 billion and US $24 billionper year in the United States alone¹⁰. Resistance to antibiotic drugsdevelops and spreads rapidly, often within a few years of first clinicaluse¹. However, the introduction of new agents to the market bypharmaceutical companies has not kept pace with the evolution ofantibiotic resistance^(1,3).

Acquired antibiotic resistance results from mutations in antibacterialtargets or from genes encoding conjugative proteins that pumpantibiotics out of cells or inactivate antibiotics¹¹. Horizontal genetransfer, which can occur via transformation, conjugative plasmids, orconjugative transposons, is a major mechanism for the spread ofantibiotic resistance genes^(12, 13). For example, Staphylococcus aureusbecame quickly resistant to sulpha drugs in the 1940s, penicillin in the1950s, and methicillin in the 1980s¹². In 2002, staphylococci developedresistance to vancomycin (the only uniformly effective antibioticagainst staphylococci) by receiving vancomycin-resistance genes viaconjugation from co-infecting Enterococcus faecalis, which itself becamecompletely resistant to vancomycin in nosocomial settings by1988^(12, 14). Some agents (e.g., ciprofloxacin) promote the horizontaldissemination of antibiotic resistance genes by mobilizing geneticelements^(15, 16). Streptococcus pneumoniae and Neisseria gonorrhoeaehave also obtained resistance to antibiotics (Morens, et al., (2004)Nature 430: 242-249). Sub-inhibitory concentrations or incompletetreatment courses can present evolutionary pressures for the developmentof antibiotic resistance¹⁷. Use of antibiotics outside of clinicalsettings, for example in livestock for the agricultural industry, hascontributed to the emergence of resistant organisms such asmethicillin-resistant staphylococci and is unlikely to abate due toeconomic reasons and modern farming practices^(12, 18). Resistance genesthat develop in non-clinical settings may be subsequently transmitted tobacterial populations which infect humans, worsening the antibioticresistance problem¹².

In addition to acquiring antibiotic-resistance genes, a smallsubpopulation of cells known as persisters can survive antibiotictreatment by entering a metabolically-dormant state^(2, 19, 20).Persister cells do not typically carry genetic mutations but ratherexhibit phenotypic resistance to antibiotics²¹. In Escherichia coli, thefraction of a population which represents persister cells increasesdramatically in late-exponential and stationary phases.Chromosomally-encoded toxins may be important contributors to thepersister phenotype but the underlying mechanisms that control thestochastic persistence phenomena are not well understood²²⁻²⁵.Persisters constitute a reservoir of latent cells that can begin toregrow once antibiotic treatment ceases and may be responsible for theincreased antibiotic tolerance observed in bacterial biofilms²⁰. Bysurviving treatment, persisters may play an important role in thedevelopment of mutations or acquisition of genes that confer antibioticresistance.

Several strategies have been proposed for controlling antibioticresistant infections. New classes of antibiotics would improve thearsenal of drugs available to fight antibiotic-resistant bacteria butfew are in pharmaceutical pipelines^(3, 26). Surveillance andcontainment measures have been instituted in government and hospitals sothat problematic infections are rapidly detected and isolated but do notaddress the fundamental evolution of resistance¹². Cycling antibioticsis one method of controlling resistant organisms but is costly and maynot be efficacious^(27, 28). Reducing the over-prescribing ofantibiotics has only moderately reduced antibiotic resistance²⁹. Effortshave been also made to lessen the use of antibiotics in farming but someuse is inevitable³⁰.

Using bacteriophage to kill bacteria has been in practice since theearly 20^(th) century, particularly in Eastern Europe^(16, 17).Bacteriophage can be chosen to lyse and kill bacteria or can be modifiedto express lethal genes to cause cell death³¹⁻³⁵. However, bacteriophagewhich are directly lethal to their bacterial hosts can also producephage-resistant bacteria in short amounts of time^(6, 7, 31, 32, 36). Inaddition to the aforementioned approaches, novel methods for designingantimicrobial drugs are becoming more important to extending thelifespan of the antibiotic era³⁷. Combination therapy with differentantibiotics or antibiotics with phage may enhance bacterial cell killingand thus reduce the incidence of antibiotic resistance, and reducepersisters³⁸⁻⁴¹. Unmodified filamentous bacteriophage have been shown toaugment antibiotic efficacy⁴². Systems biology analysis can be employedto identify pathways to target and followed by synthetic biology todevise methods to attack those pathways^(38, 43, 44).

Bacterial biofilms are sources of contamination that are difficult toeliminate in a variety of industrial, environmental and clinicalsettings. Biofilms are polymer structures secreted by bacteria toprotect bacteria from various environmental attacks, and thus resultalso in protection of the bacteria from disinfectants and antibiotics.Biofilms may be found on any environmental surface where sufficientmoisture and nutrients are present. Bacterial biofilms are associatedwith many human and animal health and environmental problems. Forinstance, bacteria form biofilms on implanted medical devices, e.g.,catheters, heart valves, joint replacements, and damaged tissue, such asthe lungs of cystic fibrosis patients⁹⁷. Bacteria in biofilms are highlyresistant to antibiotics and host defenses and consequently arepersistent sources of infection⁹⁸.

Biofilms also contaminate surfaces such as water pipes and the like, andalso render other industrial surfaces hard to disinfect⁹⁷. For example,catheters, in particular central venous catheters (CVCs), are one of themost frequently used tools for the treatment of patients with chronic orcritical illnesses and are inserted in more than 20 million hospitalpatients in the USA each year. Their use is often severely compromisedas a result of bacterial biofilm infection which is associated withsignificant mortality and increased costs.

Catheters are associated with infection by many biofilm formingorganisms such as Staphylococcus epidermidis, Staphylococcus aureus,Pseudomonas aeruginosa, Enterococcus faecalis and Candida albicans whichfrequently result in generalized blood stream infection. Approximately250,000 cases of CVC-associated bloodstream infections occur in the USeach year with an associated mortality of 12%-25% and an estimated costof treatment per episode of approximately $25,000. Treatment ofCVC-associated infections with conventional antimicrobial agents aloneis frequently unsuccessful due to the extremely high tolerance ofbiofilms to these agents. Once CVCs become infected the most effectivetreatment still involves removal of the catheter, where possible, andthe treatment of any surrounding tissue or systemic infection usingantimicrobial agents. This is a costly and risky procedure andre-infection can quickly occur upon replacement of the catheter.

SUMMARY OF THE INVENTION

Described herein are methods and compositions for the detection oftarget genes. The methods and compositions described herein can be usedto detect cells expressing any target gene, such as bacterial genes,viral genes, etc., but are particularly well suited for the detectionof, for example drug resistance genes (e.g., antibiotic resistance) inbacteria, or multi-drug resistance transporter genes in eukaryoticcells. The methods and compositions described herein further permit theselective expression of an effector gene in those cells expressing thetarget gene. In this manner, cells expressing a target gene can beselectively targeted for treatment or elimination. In certain aspects,the methods and compositions described herein permit the selectiveexpression of an agent such as a therapeutic gene product, in aspecifically targeted population of cells in an organism.

The inventors have developed a synthetic nucleic acid sensor-effectorgene circuit. In cells without a target gene, the circuit suppressese.g., effector production, but in the presence of the target gene thesuppression is subject to competition, such that the synthetic sensor isde-repressed and permits expression of the effector gene. As discussedin further detail below, preferred embodiments relate to the expressionof a reporter that permits identification of a cell or a cell-typeexpressing a target gene. Such embodiments, permit the detection ordiscrimination of cells expressing a given target gene and can beapplied to detect, for example, viral, fungal or bacterial infection orcontamination, as well as the presence of cells with specific traits,e.g., drug or antibiotic resistance.

One aspect disclosed herein relates to a method of detecting a cellexpressing a target gene by introducing to the cell a nucleic acidconstruct encoding a portion of the target gene sequence fused to aneffector gene sequence (herein referred to as the ‘sensor construct’),and a construct encoding an inhibitory nucleic acid sequence directedagainst that portion of the target gene sequence, wherein in the absenceof the target gene the inhibitory nucleic acid sequence suppresseseffector gene expression. Detection of the target gene involvesmonitoring expression of the effector gene sequence in the cell andcomparing the level of effector gene sequence expression to a referencelevel, wherein if expression of the effector gene sequence is increasedrelative to the reference level, the target gene is detected in thecell.

In one embodiment of this aspect and other aspects disclosed herein, thetarget gene is a mutant gene.

In another embodiment of this aspect and other aspects described herein,the nucleic acid construct encoding a portion of the target genesequence fused to an effector gene sequence and the construct encodingan inhibitory nucleic acid sequence are encoded on the same nucleic acidmolecule.

In another embodiment of this aspect and other aspects disclosed herein,the cell type is bacterial and the target gene is a drug resistance orvirulence gene. Alternatively, the cell type is eukaryotic/mammalian.

In another embodiment of this aspect and other aspects described hereinthe target gene is a viral gene or a multi-drug resistance transportergene.

In another embodiment of this aspect and other aspects described herein,the target gene is an oncogene or tumor associated gene. In anotherembodiment of this aspect and other aspects described herein, theeffector gene sequence encodes a reporter molecule and comprises adetectable polypeptide, for example an enzyme, a fluorescentpolypeptide, a luminescent polypeptide or an antigen. The fluorescentpolypeptide can be selected, for example, from the group consisting ofGFP, YFP, EGFP, EYFP EBFB, and fluorescent variants thereof.

Also disclosed is an engineered gene sensor comprising a nucleic acidsequence encoding a portion of a target gene sequence fused to aneffector gene sequence (herein referred to as ‘sensor construct’) andfurther comprising a construct encoding an inhibitory nucleic acidsequence directed against a portion of the target gene sequence, whereinthe presence of target gene expression in a cell permits the effectorgene expression to produce an effector polypeptide which mediatesdelivery of an agent or mediates cell killing.

Also disclosed is a method for selectively killing a cell expressing atarget gene, the method comprising the steps of introducing to the cella nucleic acid construct encoding a portion of the target gene sequencefused to an effector gene sequence, and a construct encoding aninhibitory nucleic acid sequence directed against a portion of thetarget gene sequence, wherein in the absence of target gene expressionthe inhibitory nucleic acid sequence suppresses expression of the effectgene; wherein the presence of target gene expression in the cell permitseffector gene expression to produce an effector polypeptide, and whereinexpression of the effector polypeptide mediates cell killing.

In one embodiment, the effector gene sequence encodes a polypeptide thatmediates cell killing. As non-limiting examples, the effector genesequence can encode a hok or CcdB polypeptide in a bacterial cell.Alternatively, the effector gene sequence can be selected from, forexample apoptotic gene products, or anti-angiogenic gene products. Inanother embodiment, the effector gene encodes for an RNA interferencemolecule.

Another aspect of the invention disclosed herein is a system fordetecting a target gene, the system comprising a nucleic acid constructencoding a portion of the target gene sequence fused to an effector genesequence (sensor construct); and a construct encoding an inhibitorynucleic acid sequence that inhibits the sensor construct expression,wherein both nucleic acid constructs are comprised by the same nucleicacid molecule. In one embodiment, the nucleic acid molecule comprises avector, for example a plasmid vector, a viral vector or a bacterialvector. In one embodiment, the effector gene sequence encodes apolypeptide that mediates cell killing. Alternatively, the effector genesequence can encode an RNA interference molecule, which may or may notmediate cell killing. In one embodiment the effector is an apoptoticgene, for example a caspase. In another embodiment, the effector altersangiogenic pathways and expression of angiogenic genes (e.g., VEGF,PDGF, FGF and Akt). In another embodiment, the effector gene encodes adetectable protein such as a fluorescent protein.

Another aspect of the invention relates to a kit for detecting a targetgene, the system comprising (a) a nucleic acid construct encoding aportion of the target gene sequence fused to an effector gene sequence,(b) a construct encoding an inhibitory nucleic acid sequence thatinhibits the expression of the target gene portion/effector fusion,wherein the nucleic acid constructs of (a) and (b) are comprised by thesame nucleic acid molecule and (c) packaging materials therefor.

Definitions

As used herein, the term ‘portion’ when used in relation to a targetgene sequence denotes a length of nucleic acid sequence sufficient tomediate inhibition by an inhibitory nucleic acid sequence as describedherein.

As used herein, the term ‘effector’ designates a nucleic acid sequenceencoding a polypeptide or RNA interference molecule, which uponexpression produces a reporter protein, a therapeutic agent or mediatescell killing.

As used herein, the terms ‘inhibits’, ‘inhibiting’ and ‘inhibition’signify a reduction in a measurable parameter (e.g., mRNA or proteinlevel) of target gene expression by at least 10%, e.g., at least 15%, atleast 20%, at least 30%, at least 40%, at least 50%, at least 60%, atleast 70%, at least 80%, at least 90%, at least 95%, at least 99% oreven 100% (ie target gene expression is inhibited completely), relativeto a reference.

As used herein, the terms ‘suppresses’, ‘suppressing’ and ‘suppression’denote maintenance of effector gene expression at levels that are within10% of reference levels. ‘Reference levels’ denotes a measure ofreporter gene expression compared to a reference. The term ‘Reference’as described herein denotes a cell which does not express the targetgene and represents the baseline level of reporter gene or effector geneexpression, e.g., in a cell containing a sensor construct and aninhibitory construct as described herein.

As used herein, the term ‘increased’ when used in relation to geneexpression or another measurable parameter means detection of e.g.,effector gene expression at levels at least 10% higher than referencelevels, e.g., at least 20%, at least 30%, at least 40%, at least 50%, atleast 60%, at least 70%, at least 80%, at least 90%, at least 100%, atleast 2-fold, at least 5-fold, at least 10-fold, at least 100-fold ormore.

As used herein, the term ‘drug-resistance gene’ denotes a gene whichupon expression allows cells to survive in the presence of a drug whichis toxic to cells lacking expression of the gene.

As used herein, the term ‘virulence gene’ signifies a gene which uponexpression renders a cell more pathogenic than its counterparts whichlack expression of the virulence gene.

As used herein, the term ‘tumor associated gene’ refers to a gene whichupon expression produces a tumor antigen in tumorogenic cells. The term‘tumor antigen’ is used herein to refer to proteins present only ontumor cells (tumor specific antigens) as well as those present on normalcells but expressed preferentially on tumor cells (tumor associatedantigens).

As used herein, the term “mutant gene” refers to a gene that differsfrom a wild-type gene in sequence or function. A “mutant gene” willgenerally result in a protein or polypeptide that differs by at leastone amino acid relative to a wild-type reference sequence, and canencompass differences in, for example, at least two, at least three, atleast four, at least five, at least 10 amino acids, etc. relative to areference sequence. A “mutant gene” will generally, but not always,exhibit altered function relative to a wild-type gene. For example, theproduct of a mutant gene can exhibit enhanced (e.g., by at least 5%, atleast 10%, at least 15%, at least 20%, at least 30%, at least 40%, atleast 50%, at least 1 fold, at least 2 fold, or more) or decreased(e.g., by at least 5%, at least 10%, at least 15%, at least 20%, atleast 30%, at least 40%, at least 50%, at least 60%, at least 70%, atleast 80%, at least 90% or more, including 100%, (i.e., no function)function relative to a wild-type gene. The term “mutant gene” alsoencompasses a gene which encodes a wild-type polypeptide but which, dueto a change in a regulatory sequence, is expressed (or not expressed)inappropriately, e.g., at least 20% higher or lower, at least 30% higheror lower, at least 40% higher or lower, at least 50% higher or lower, atleast 60% higher or lower, at least 70% higher or lower, at least 80%higher or lower or at least 90% higher or lower than the wild type gene.

As used herein, the term ‘selectively killing’ means a given treatmentor process which results in the killing of cells expressing a targetgene to the substantial exclusion of cells that do not express thetarget gene. ‘Substantial exclusion’ as used in this context means that≦30% of cells killed did not express the target gene, or alternatively,that ≦25%, ≦20%, ≦15%, or ≦10% of cells killed did not express thetarget gene, preferably ≦5% of cells killed did not express the targetgene; more preferably ≦1%.

As used herein, the term ‘mediates cell killing’ denotes the productionof an effector that promotes cell death by increasing susceptibility ofcells to drug treatments, activation of an endogenous cell deathpathway, suppression of an endogenous growth pathway, or an effectorthat is directly toxic to the cell e.g., the bacterial toxin MazF; aneffector that mediates cell killing can be a polypeptide or in otherinstances a nucleic acid, for example an RNA interference molecule.

An “RNA interference molecule” as used herein, is defined as any agentwhich interferes with or inhibits expression of a target gene or genomicsequence by RNA interference (RNAi). Such RNA interfering agentsinclude, but are not limited to, nucleic acid molecules including RNAmolecules which are homologous to the target gene or genomic sequence,or a fragment thereof, short interfering RNA (siRNA), short hairpin orsmall hairpin RNA (shRNA), microRNA (miRNA) and small molecules whichinterfere with or inhibit expression of a target gene by RNAinterference (RNAi).

“RNA interference (RNAi)” is an evolutionally conserved process wherebythe expression or introduction of RNA of a sequence that is identical orhighly similar to a target gene results in the sequence specificdegradation or specific post-transcriptional gene silencing (PTGS) ofmessenger RNA (mRNA) transcribed from that targeted gene⁴⁵, therebyinhibiting expression of the target gene. In one embodiment, the RNA isdouble stranded RNA (dsRNA). This process has been described in plants,invertebrates, and mammalian cells. In nature, RNAi is initiated by thedsRNA-specific endonuclease Dicer, which promotes processive cleavage oflong dsRNA into double-stranded fragments termed siRNAs. siRNAs areincorporated into a protein complex (termed “RNA induced silencingcomplex,” or “RISC”) that recognizes and cleaves target mRNAs. RNAi canalso be initiated by introducing nucleic acid molecules, e.g., syntheticsiRNAs or RNA interfering agents, to inhibit or silence the expressionof target genes. As used herein, “inhibition of target gene expression”includes any decrease in expression or protein activity or level of thetarget gene or protein encoded by the target gene as compared to asituation wherein no RNA interference has been induced. The decreasewill be of at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95% or99% or more as compared to the expression of a target gene or theactivity or level of the protein encoded by a target gene which has notbeen targeted by an RNA interference molecule. The terms “RNAinterference” and “RNA interference molecule” as they are used hereinare intended to encompass those forms of gene silencing mediated bydouble-stranded RNA, regardless of whether the RNA interfering agentcomprises an siRNA, miRNA, shRNA or other double-stranded RNA molecule.

“Short interfering RNA” (siRNA), also referred to herein as “smallinterfering RNA” is defined as an RNA agent which functions to inhibitexpression of a target gene, e.g., by RNAi. An siRNA may be chemicallysynthesized, may be produced by in vitro transcription, or may beproduced within a host cell. In one embodiment, siRNA is a doublestranded RNA (dsRNA) molecule of about 15 to about 40 nucleotides inlength, preferably about 15 to about 28 nucleotides, more preferablyabout 19 to about 25 nucleotides in length, and more preferably about19, 20, 21, 22, or 23 nucleotides in length, and may contain a 3′ and/or5′ overhang on each strand having a length of about 0, 1, 2, 3, 4, or 5nucleotides. The length of the overhang is independent between the twostrands, i. e., the length of the overhang on one strand is notdependent on the length of the overhang on the second strand. Preferablythe siRNA is capable of promoting RNA interference through degradationor specific post-transcriptional gene silencing (PTGS) of the targetmessenger RNA (mRNA).

As used herein, the term ‘alters’ denotes an increase or decrease ineffector gene expression of at least 10% e.g., at least 15%, at least20%, at least 30%, at least 40%, at least 50%, at least 60%, at least70%, at least 80%, at least 90%, at least 95%, at least 99% or even 100%or more. Where the alteration is a decrease 100% means complete orsubstantially complete inhibition of effector gene expression. By‘substantially complete inhibition’ is meant that expression is notdetected above background by standard methods. Where the alteration isan increase the term further encompasses greater than 100% increase,e.g., 2-fold, 5-fold, 10-fold, 100-fold or further increase over abaseline or reference level.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1. A schematic depiction of the paired-termini design which extendsthe half-life of an inhibitory nucleic acid sequence upon introductionto a cell⁴⁶ (Incorporated from reference quoted herein). The nucleicacid sequence depicted is SEQ ID NO: 1.

FIG. 2. A diagram depicting the expression of a reporter molecule (e.g.,GFP) in the absence and presence of a target gene.

FIG. 3. A schematic diagram depicting an engineered gene sensor designwhich allows the detection of a target gene but not other genesexpressed in the cell.

FIG. 4. A graph depicting the increase in expression of a reportermolecule compared to reference levels as indicated by the increase influorescence intensity in cells expressing the target gene after (a) 4.5h, (b) 5.5 h and (c) 6.5 h of cell growth.

FIG. 5. A schematic depiction of a gene sensor molecule design thatdetects a target gene and effects the degradation of a repressormolecule, which in turn permits effector gene expression. (For furthersupport see⁴⁷ incorporated from reference quoted herein)

FIG. 6. Example nucleic acid sequences that encode for bacterialantibiotic resistance. Chloramphenicol acetyltransferase, SEQ ID NO: 2;Tetracycline resistance sequence, SEQ ID NO: 3; kanamycin resistancesequence, SEQ ID NO: 4; ampicillin resistance sequence, SEQ ID NO: 5;vancomycin resistance genes vanA (SEQ ID NO: 6), vanH (SEQ IDNO: 7) andvanX (SEQ ID NO: 8); and methicillin resistance (SEQ ID NO: 9).

FIG. 7. A model for estimating selective sensing of a target gene.

Table 1 includes the nucleic acid sequence (SEQ ID NO: 10) of anexemplary gene sensor comprising a nucleic acid vector used herein andillustrates the sequence of the pZE21s1-cat plasmid. The PT7 antisensestem loop structure is illustrated by bold and underlined letters. DNAcoding for the antisense RNA to cat is denoted with italics and largecase letters. The DNA sequence for the cat-gfp fusion polypeptide isdenoted by italics/lowercase letters and underlined large case letters,respectively.

Table 2 includes a list of potential target genes involved in enhancedvirulence of bacteria.

DETAILED DESCRIPTION

Described herein are methods and compositions for the detection oftarget genes. The methods and compositions described can be used todetect cells expressing any target gene, but are particularly wellsuited for the detection of, for example drug resistance genes (e.g.,antibiotic resistance) in bacteria, or multi-drug resistance transportergenes in eukaryotic cells. The methods and compositions described hereinfurther permit the selective expression of an effector gene in thosecells expressing the target gene. In this manner, cells expressing atarget gene can be selectively targeted for treatment or elimination. Incertain aspects, the methods and compositions described herein permitthe selective expression of an agent such as a therapeutic gene product,in a specifically targeted population of cells in an organism.

The following describes materials and methods useful for the practice ofthe invention described herein.

Gene Sensor System

The gene sensor described herein comprises a nucleic acid vector, whichencodes for (a) a portion of the target gene, fused to an effector genesequence (sensor construct); and (b) an inhibitory nucleic acid moleculedirected against the sensor construct. Together these two sequences forma gene sensor coupled to an effector molecule. Upon expression of thesensor construct and the inhibitory nucleic acid molecule in the samecell, the target gene portion of the sensor construct transcript bindsto the inhibitory nucleic acid sequence. This binding suppressesexpression of the effector gene portion of the sensor construct andretains effector molecule expression at a level comparable to thereference or control level. However, if the target gene is alsoexpressed in the cell, the mRNA sequence encoding the target geneproduct competes with the sensor construct transcript for binding to theinhibitory nucleic acid molecule. As the system approaches equilibrium,the target gene mRNA will displace the sensor construct transcript fromthe inhibitory nucleic acid molecule. Displacement of the sensorconstruct transcript releases the inhibition of the effector portion ofthe sensor construct, thereby allowing expression of the selectedeffector molecule.

The effector molecule can produce a reporter molecule that would beuseful for detection of cells in a population that express the targetgene. The reporter molecule can mediate fluorescence, a colorimetricsignal or confer resistance to a drug to allow one to distinguish cellsexpressing the target gene from cells lacking expression.

Alternatively, the effector can selectively mediate cell killing. Theeffector can encode for a polypeptide toxic to the cell or a polypeptidethat activates cell death pathways (e.g., apoptotic death pathways) orinterferes with cell growth pathways (e.g., angiogenic pathways).Effector genes can include, for example caspases or FLK(VEGF trap) orany of those described herein below.

FIG. 1 depicts one example of an inhibitory nucleic acid sequence in theform of an antisense RNA directed against the target gene. In thisexample, paired termini technology ⁴⁶ is used to extend the half-life ofinhibition in the cell. The antisense RNA in this example is directedagainst the target gene transcript but also binds to the target geneportion of the sensor construct transcript. FIG. 2 is a schematicdepiction of expression of an exemplary reporter molecule (GFP) in theabsence and presence of target gene expression in a cell. In the absenceof the target gene transcript the sensor construct transcript isinhibited by binding of its transcript to the inhibitory nucleic acidconstruct. Upon target gene expression, the encoding mRNA competes withthe target gene portion of the sensor construct transcript for bindingto the inhibitory nucleic acid transcript. This displacement of thesensor construct transcript permits the effector gene to be expressedand in this embodiment the fluorescence of GFP can be detected as a readout of target gene expression. It is important to note that theinhibitory nucleic acid molecule need not necessarily be an antisensemolecule. It could also be, for example, an RNA interference-typemolecule that mediates silencing of expression of the sensor construct,e.g., by cleavage of the sensor construct's RNA transcript in, forexample a eukaryotic cell. Ribozymes specific for a portion of a targetgene transcript are also contemplated.

Target Genes

One aspect of the present invention is detection of a target gene in acell. Any gene can be a target gene. A target gene is preferably a geneof interest to allow detection and optionally subsequent treatment ofdisease. In one embodiment the target gene is a mutant gene. This aspectof the invention allows flexibility in the choice of target to extendthe application to various disease states.

A target gene is selected based on the genetic characteristics of thecell to be detected (e.g., bacterial vs. eukaryotic/mammalian), thedisease to be detected or treated (e.g., antibiotic resistant bacterialinfections, cancer) or a disease related to the lack of or excess of oneor more protein targets (e.g., Cystic Fibrosis, Adrenoleukodystrophy).For example, in the treatment of an antibiotic-resistant bacterialinfection, a target gene that confers resistance to a particular agentwould be selected in order to allow detection of antibiotic resistantcells in a population. Some examples of target genes which conferantibiotic resistance to bacteria comprise β-lactamase genes, penicillinbinding site (PBP) genes, and multi-drug efflux pump genes (e.g.,members of the RND, MFS, SMR and MATE families of multi-drug effluxpumps; specific examples include but are not limited to MExAB-OprM,MexCD-OprJ, MexEF-OprN, MesHI-OprD, MexJK-OprM, MexXY-OprM, AdeABC,SmeABC, SmEDEG, CeoAB-OpcM, AmrAB-AprA, AcrAB-TolC, MepA, EmeA, Lsa andPmrA)⁴⁸. Other non-limiting examples of antibiotic resistance genesinclude LmrA, AcrB, AcrA, TolC, Sav1866, EmrE, EmrD, LmrP, BmrR, VanS,VanR, VncS, VncR, RprX, RprY, cat, TetR, kan, bla, vanA/vanH/vanX, mecR,vanB/vanH/vanX, qnr, McbG, ermB, ermC, ermG, and QacR. (For supportsee⁴⁹⁻⁵⁴)

Another example of a target gene in bacteria is a virulence gene, whichupon expression can confer increased pathogenicity of a cell population.By “increased” in this context is meant at least 10%, at least 20%, atleast 30%, at least 40%, at least 50%, at least 60%, at least 70%, atleast 80%, at least 90%, at least 2 fold, at least 5-fold or moreincreased relative to the pathogenicity of a reference pathogen, e.g., apathogen that differs from the subject pathogen essentially only inlacking that virulence gene. Virulence genes produce factors whichincrease the ability of bacteria to induce illness in its host.Increased virulence can result from the following: (a) the enhancedexpression of adhesion molecules that allow host uptake of bacteria, (b)the production of proteins that allow bacteria to survive in hostileenvironments (e.g., extreme pH of the gastrointestinal system), (c) theability to disrupt host cell membranes to increase speed of invasion,(d) the production of host immune-suppressive proteins, and (e) theproduction of toxins secreted by bacteria that can cause tissue damage.Some non-limiting examples of virulence genes in bacteria are shown inTable 2 and can be found, for example on the world wide web atjenner.ac.uk/BacBix3/PPprints.htm.

Other non-limiting examples of virulence gene targets, but are notlimited, to FimC, FimD, FimH, PapC, PapD, PapG, PrsD, PrsC, PrsG, SfaE,SfaF, SfaS, HifB, HifC, HifE, FimB, PefD, PefC, LpfB, LpfC, MrkB, MrkC,MyfB, MyfC, MrkD, CooB, CooC, CooD, CsgA, CsgB, CsgE, CsgF, CsgG, AgfB,AgfA, PilC, pilin, AgrC, AgrA, AlgD, AlgR, RcsC, RcsB, BvgS, BvgA, PhoQ,and PhoP^(52,55). Alternatively, a target gene can be a gene expressedby a eukaryotic/mammalian cell. An example of this embodiment includesthe target genes expressed by cancerous tumors, wherein the target geneis, for example, a mutant gene which confers drug resistance or aproliferative advantage to a particular cell population. For example,the mammalian P-glycoprotein, also known as ABCB1, is an activetransporter that pumps a variety of toxic agents used in chemotherapyout of the cell. Inhibition of mammalian P-glycoprotein could conferincreased sensitivity of tumors to existing chemotherapeutic agents. Thetarget could alternatively be a tumor antigen or tumor associatedantigen that permits targeting of cells expressing them with atherapeutic molecule. Other non-limiting examples of tumor associatedgenes include, for example, those described in U.S. Pat. No. 6,635,476,which is incorporated herein by reference⁵⁶. Examples include but arenot limited to alfa-fetoprotein (AFP), C-reactive protein (CRP), cancerantigen-50 (CA-50), cancer antigen-125 (CA-125) associated with ovariancancer, cancer antigen 15-3 (CA15-3) associated with breast cancer,cancer antigen-19 (CA-19) and cancer antigen-242 associated withgastrointestinal cancers, carcinoembryonic antigen (CEA), carcinomaassociated antigen (CAA), chromogranin A, epithelial mucin antigen(MC5), human epithelium specific antigen (HEA), Lewis(a)antigen,melanoma antigen, melanoma associated antigens 100, 25, and 150,mucin-like carcinoma-associated antigen, multidrug resistance relatedprotein (MRPm6), multidrug resistance related protein (MRP41), Neuoncogene protein (C-erbB-2), neuron specific enolase (NSE),P-glycoprotein (mdr1 gene product), multidrug-resistance-relatedantigen, p170, multidrug-resistance-related antigen, prostate specificantigen (PSA), CD56, and NCAM. Further non-limiting examples of oncogenetarget genes include those described by Croce, 2008, New England Journalof Medicine, 358:502-511 (and in the Supplementary data of the same),which is incorporated herein by reference⁵⁷. Examples include but arenot limited to ERBB2, EGFR, VEGF, ABL, PDFR, KIT, FLT3, MYC, RAS, BCL2,v-myc, N-MYC, L-MYC, v-myb, v-fos, v-jun, v-ski, v-rel, v-ets-1,v-ets-2, v-erbA1, v-erbA2, MDM2, ALL1, v-sis, int2, KS3, HST, v-fms,v-KIT, v-ros, MET, TRK, RET, mas, SRC, v-yes, v-fgr, v-fes, ABL, H-RAS,K-RAS, N-RAS, BRAF, gsp, gip, Dbl, Vav, v-mos, v-raf, pim-1, v-crk andc-MYC.

Another example of target gene detection in eukaryotic cells relates tolatent viral infections, which persist due to the virus' ability toremain dormant in a cell it has infected or its ability to incorporateits viral genome into the eukaryotic genome of the cell. Exampleinfections include herpes simplex virus, Human Immunodeficiency Virus(HIV), Epstein-barr virus, Varicella-zoster virus and human papillomavirus among others. Examples of target genes involved in herpes simplexvirus latency in neurons include, but are not limited to, immediateearly gene (IE), latency associated transcripts (LATS), and histonedeacetylases (HDACs). Some non-limiting examples of HIV latent genes inCD4+ T-cells include APO-BEC3G, CCR5, HDACs, Sp1, LEF-1, COUP-TF, YYI,ETS-1, USF, NF-kB, AP-1, NFAT, tat, rev, nev and P-TEFb. Other genesrelated to viral latency are known to those skilled in the art. (Forfurther support please see⁵⁸⁻⁶¹)

Effectors

Disclosed herein are methods for detecting target genes that involve thesubsequent activation of effector gene sequences. Effector genes encodefor polypeptides with a desired function in the cell. Frequently, aneffector will be exogenous to the cell in which it is expressed.However, it is also contemplated that an effector, in some situations,may be an endogenous polypeptide expressed, for example at a higherlevel in the host cell.

In one embodiment, the effector gene sequence encodes for a reportermolecule that is expressed in the presence of a target gene to allowdetection of cells in a population that carry the target gene. Areporter is encoded by a molecule that can be attached to a nucleic acidof interest (e.g., a portion of the target gene) that allows theexpression of the nucleic acid to be easily identified and/or quantifiedin a cell. In addition, reporter molecules can also be used forselectively identifying cells in a population by e.g., allowing forgrowth on a medium or in the presence of an agent that would normallynot permit growth of the cell population. A reporter molecule can allowthe user to determine if the sensor construct is being expressed in thecell and in some cases can be used to quantify the level of expressioncompared to a reference level. Reporters can be enzymes which producelight (e.g., luciferase), colored products (e.g., β-galactosidase), orconfer antibiotic resistance (e.g., CAT). Reporters can be enzymes thatcatalyze any reaction that yields a detectable product when acting on asubstrate. In addition, reporters can also be antigens that are detectedby binding to specific antibodies using standard methods (e.g., FACS,Western blotting, dot blotting, immunohistochemistry or ELISA).Reporters can also be fluorescent polypeptides that upon stimulationwith a certain wavelength of light, will emit fluorescence at a separatewavelength (e.g., GFP and other fluorescent variants described herein).

In addition to detection where the effector is a reporter, anotherembodiment relates to the selective killing of cells mediated byeffector gene sequences. In this manner, gene sensors as describedherein allow detection of cells expressing a target gene and couplesthis detection to the production of a polypeptide that mediates cellkilling. The mediation of cell killing can be direct, such that theeffector encodes for expression of a toxin, or indirect, for example bymaking a cell more susceptible to an agent or drug.

The mediation of cell killing in bacteria can occur by influencingbacterial programmed cell death pathways that exist in the form of atoxin/antitoxin pair. The toxin is inactivated by antitoxinsequestration under normal growth conditions. However, some cellularstresses (e.g., thymine starvation or antibiotic treatment) can causethe loss of the antitoxin protein, thereby releasing the toxin. Thetoxin initiates an irreversible cell death cascade by virtue of itsendoribonucleolytic effect (e.g., cleavage) on mRNA molecules. The genesinvolved in the toxin/anti-toxin pair are good candidates for the choiceof effector in the described system and effectors can include, but arenot limited to the toxins mazF, ccdB, relE, parE, higB, doc, or vapC, orthe inhibition of anti-toxin genes such as masE, skfE, skfF, relB, parD,higA, phd, and vapB. (Support for this concept is provided by^(62,63))

The mediation of mammalian cell killing by the effector can also resultfrom activation of a cell death pathway. Currently there are three knowntypes of programmed cell death in mammalian cells: (a) apoptosis, (b)necrosis, and (3) autophagy. While the activation of any of thesepathways would mediate cell killing, it is preferable to utilize amember of the apoptotic cell death pathway since it remains the mosthighly characterized. Apoptosis can result from two different activationpathways, termed the ‘extrinsic or death receptor pathway’ and‘intrinsic or stress pathway’. The extrinsic pathway is activated byextracellular stimuli that bind to surface receptors (e.g., TRAILreceptor), whereas the intrinsic pathway responds to intracellularstimuli (e.g., mitochondrial membrane permeabilization or cytochrome crelease). Both pathways converge downstream and promote activation ofthe caspase family of proteases. Active caspases proteolytically cleavehundreds of intracellular proteins involved in cell survival, DNA repairand replication of the cell. Thus, for example caspases, especially theinitiator caspases such as caspase-2, caspase-8, caspase-9, andcaspase-10 are appropriate for use as an effector in this system.

There exists a balance of anti-apoptotic and apoptotic stimuli in a cellat any given time. Therefore, apoptosis can be influenced either byactivating a pro-apoptotic pathway or by inhibiting an anti-apoptoticpathway. Activation can occur, for example, by the effector expressing apro-apoptotic gene or by inhibiting an anti-apoptotic gene, e.g., by RNAinterference, or by intracellular expression of an antibody fragment orbinding partner that inactivates the anti-apoptotic gene or its product.For example, the Bcl-2 family of proteins contain pro-apoptotic proteinssuch as Bax and Bak, which promote cytochrome c release frommitochondria. Activation of Bax and Bak can mediate cell death byapoptosis. Alternatively, the Bcl-2 family also includes anti-apoptoticmembers including Bcl-2, Bcl-xL, Bcl-w, Mcl-1 and A1, which can beinhibited by the use of an appropriate effector in the system describedherein. Similarly, members of the ‘inhibitor of apoptosis’ (IAP) familyincluding, but not limited to, DIAP, XIAP, Smac/Diablo, Omi/HtrA2,GSPT1/eRF3, and cIAP are potential effector targets that could beinhibited in cells e.g., by RNA interference. Other options of effectorsfrom the extrinsic or death receptor pathway include, but are notlimited to death receptors (e.g., TRAIL receptor), Fas activated deathdomain (FADD), and caspase 8. Some further non-limiting examples of theintrinsic/stress pathway include BH3 domain containing proteins(specific examples include BIM, BID, PUMA, BAD, HRK, BIK and NOXA),Apaf1, caspase-3, caspase-6, and caspase-7. (For further support pleasesee ^(64,65))

In another embodiment, the effector gene sequence can be used to replacea polypeptide that is missing in a cell. For example, the absence of thechloride channel CFTR is responsible for the manifestation of cysticfibrosis and could be replaced using the technology described herein. Analternative example is the absence of the ABC transporter isoform D1,which manifests as adrenoleukodystrophy. Thus ABCD1 is another possibleeffector for use in this system. Any other defective or mutant proteincould potentially be replaced using this approach. An advantage of thisapproach is that it can specifically target cells expressing the mutantgene for replacement of the corrective gene product.

In an alternative embodiment, the effector gene sequence can encode fora repressor of a polypeptide. For example, a repressor can be designedas described by Bartsevich et al, 2000 Molecular Pharmacology 58:1-10incorporated herein by reference⁶⁶, which for example would mediate theselective repression of a multi-drug resistance tranporter. Similarly,the effector gene sequence can encode for a transcription factor thatwill repress the production of polypeptides, for example those involvedin drug resistance in both bacterial and mammalian cells. Thisrepression can be a result of enhanced production of antisense RNA,siRNA, shRNA, or miRNA by the transcription factor that will produceinhibition of target expression. For further support on siRNA and shRNAstrategies see, for example de Fougerolles, A., et al (2007); and Xiang,S. et al (2006), which are incorporated herein by reference ^(67,68))

In another embodiment the effector gene sequence encodes for an RNAinterference molecule that can inhibit or activate any of a number ofdesired cell pathways, for example, cellular growth, angiogenic orapoptotic pathways.

In another embodiment the effector gene sequence can encode for e.g.,porins that enhance antibiotic removal from the cell as described byCeccarelli et al, 2004⁵⁴.

In another embodiment the effector gene sequence can encode factors thatincrease death or susceptibility to e.g., antibiotics⁶⁹⁻⁷¹.

In another embodiment the effector gene sequence can encode for e.g., asoluble receptor or antibody that binds to and inhibits a target genepolypeptide.

Preparation of a Sensor Construct System

The target gene of interest and the effector gene sequence are chosenbased on a rational approach designed to target a specific cell ordisease state. Most often, a portion of the target gene transcriptcomprising a minimum of 10 base pairs to a maximum length of the entiretarget gene sequence is ligated to the effector gene sequence such thatthe target gene sequence and effector gene sequence form a seamlesspolypeptide. It is important to choose a portion of target gene sequencefor the sensor construct that is not capable of producing a functionaltarget protein from the sensor construct. In order to prevent functionaltarget protein expression from the sensor construct, it is preferable touse a smaller, e.g., incomplete, portion of the target gene to fuse tothe effector sequence. Other approaches to prevent functional proteinexpression by the sensor construct involve altering the nucleic acidsequence of the target gene portion of the sensor construct and include,but are not limited to, the insertion or loss of a base pair to producea non-coding frame shift, mutation of essential domains (e.g., ATPbinding domains of kinases), or production of a missense polypeptide. Itis also important to note that the portion of the target gene transcriptdoes not have to be identical in sequence to the target gene and caninstead have a degree of mismatch. This degree of mismatch can bealtered as desired in the design of the sensor construct in order tochange binding characteristics as described herein (see ‘Mediation ofinhibition’ and ‘RNA interference section’).

The other molecule of the gene sensor, termed the inhibitory nucleicacid construct, can also be tailored to the needs of the user and choiceof the target gene. The inhibitory nucleic acid sequence utilized tobind the sensor construct is determined by, for example, the species ofcell to be targeted. Bacterial cells rapidly degrade foreign nucleicacids, so in order to extend the half-life of the inhibitory nucleicacid transcript sequence in the cell, the construct is inserted, forexample, into a paired termini system described by Nakashima et al⁴⁶.The inhibitory nucleic acid sequence can be inserted, for example, byligation into the paired termini design⁴⁶ at the top of the stem. Theinhibitory nucleic acid sequence can be more flexible in mammaliancells, which permits the use of RNA interference molecules, e.g., siRNA,shRNA, and miRNA.

In one embodiment, the inhibitory nucleic acid sequence as well as thetarget gene-effector gene sequence encoding for the fusion polypeptideare encoded on the same nucleic acid molecule. In an alternativeembodiment, the inhibitory nucleic acid sequence as well as the targetgene-effector gene sequence encoding for the fusion polypeptide areencoded on two separate nucleic acid molecules, allowing for greatersensitivity of detection. The use of two constructs permits the changesin effector concentration to be regulated tightly by altering the doseof the inhibitory nucleic acid molecule and the sensor construct inrelation to each other.

RNA Interference

RNA interference (RNAi) is an evolutionally conserved process wherebythe expression or introduction of RNA of a sequence that is identical orhighly similar to a target gene results in the sequence specificdegradation or specific post-transcriptional gene silencing (PTGS) ofmessenger RNA (mRNA) transcribed from that targeted gene⁷², therebyinhibiting expression of the target gene. In one embodiment, the RNA isdouble stranded RNA (dsRNA). This process has been described in plants,invertebrates, and mammalian cells. In nature, RNAi is initiated by thedsRNA-specific endonuclease Dicer, which promotes processive cleavage oflong dsRNA into double-stranded fragments termed siRNAs. siRNAs areincorporated into a protein complex (termed “RNA induced silencingcomplex,” or “RISC”) that recognizes and cleaves target mRNAs. RNAi canalso be initiated by introducing nucleic acid molecules, e.g., syntheticsiRNAs or RNA interfering agents, to inhibit or silence the expressionof target genes.

In the invention disclosed herein, the effector sequence can encode anRNA interference molecule to silence one or more genes in a target cell.Alternatively, the inhibitory nucleic acid construct can encode for anRNA interference molecule to silence the sensor construct transcript. Insome cases both the effector sequence and inhibitory nucleic acidconstruct can encode RNA interference molecules, for example, siRNA,miRNA, shRNA or other double stranded RNA molecule.

Short interfering RNA (siRNA) is defined as an RNA agent which functionsto inhibit expression of a target gene, e.g., by RNAi. An siRNA may bechemically synthesized, may be produced by in vitro transcription, ormay be produced within a host cell. In one embodiment, siRNA is a doublestranded RNA (dsRNA) molecule of about 15 to about 40 nucleotides inlength, preferably about 15 to about 28 nucleotides, more preferablyabout 19 to about 25 nucleotides in length, and more preferably about19, 20, 21, 22, or 23 nucleotides in length, and may contain a 3′ and/or5′ overhang on each strand having a length of about 0, 1, 2, 3, 4, or 5nucleotides. The length of the overhang is independent between the twostrands, i. e., the length of the overhang on one strand is notdependent on the length of the overhang on the second strand. Preferablythe siRNA is capable of promoting RNA interference through degradationor specific post-transcriptional gene silencing (PTGS) of the targetmessenger RNA (mRNA).

siRNAs also include small hairpin (also called stem loop) RNAs (shRNAs).In one embodiment, these shRNAs are composed of a short (e.g., about 19to about 25 nucleotide) antisense strand, followed by a nucleotide loopof about 5 to about 9 nucleotides, and the analogous sense strand.Alternatively, the sense strand may precede the nucleotide loopstructure and the antisense strand may follow.

The target gene or sequence of the RNA interfering agent may be acellular gene or genomic sequence, e.g., those described herein astarget genes. An siRNA may be substantially homologous to the targetgene or genomic sequence, or a fragment thereof. As used in thiscontext, the term “homologous” is defined as being substantiallyidentical, sufficiently complementary, or similar to the target mRNA, ora fragment thereof, to effect RNA interference of the target. Inaddition to native RNA molecules, RNA suitable for inhibiting orinterfering with the expression of a target sequence include RNAderivatives and analogs. Preferably, the siRNA is identical to itstarget.

The siRNA preferably targets only one sequence. Each of the RNAinterfering agents, such as siRNAs, can be screened for potentialoff-target effects by, for example, expression profiling. Such methodsare known to one skilled in the art and are described, for example, inJackson et al. (2003)⁷³. In addition to expression profiling, one mayalso screen the potential target sequences for similar sequences in thesequence databases to identify potential sequences which may haveoff-target effects. For example, according to Jackson et al.⁷³ 15, orperhaps as few as 11 contiguous nucleotides, of sequence identity aresufficient to direct silencing of non-targeted transcripts. Therefore,one may initially screen the proposed siRNAs to avoid potentialoff-target silencing using the sequence identity analysis by any knownsequence comparison methods, such as BLAST.

siRNA sequences are chosen to maximize the uptake of the antisense(guide) strand of the siRNA into RISC and thereby maximize the abilityof RISC to target human GGT mRNA for degradation. This can beaccomplished by scanning for sequences that have the lowest free energyof binding at the 5′-terminus of the antisense strand. The lower freeenergy leads to an enhancement of the unwinding of the 5′- end of theantisense strand of the siRNA duplex, thereby ensuring that theantisense strand will be taken up by RISC and direct thesequence-specific cleavage of the target gene transcript.

siRNA molecules need not be limited to those molecules containing onlyRNA, but, for example, further encompasses chemically modifiednucleotides and non-nucleotides, and also include molecules wherein aribose sugar molecule is substituted for another sugar molecule or amolecule which performs a similar function. Moreover, a non-naturallinkage between nucleotide residues can be used, such as aphosphorothioate linkage. The RNA strand can be derivatized with areactive functional group of a reporter group, such as a fluorophore.Particularly useful derivatives are modified at a terminus or termini ofan RNA strand, typically the 3′ terminus of the sense strand. Forexample, the 2′-hydroxyl at the 3′ terminus can be readily andselectively derivatized with a variety of groups.

Other useful RNA derivatives incorporate nucleotides having modifiedcarbohydrate moieties, such as 2′O-alkylated residues or 2′-O-methylribosyl derivatives and 2′-O-fluoro ribosyl derivatives. The RNA basesmay also be modified. Any modified base useful for inhibiting orinterfering with the expression of a target sequence may be used. Forexample, halogenated bases, such as 5-bromouracil and 5-iodouracil canbe incorporated. The bases may also be alkylated, for example,7-methylguanosine can be incorporated in place of a guanosine residue.Non-natural bases that yield successful inhibition can also beincorporated.

The most preferred siRNA modifications include 2′-deoxy-2′-fluorouridineor locked nucleic acid (LAN) nucleotides and RNA duplexes containingeither phosphodiester or varying numbers of phosphorothioate linkages.Such modifications are known to one skilled in the art and aredescribed, for example, by Braasch et al., (2003)⁷⁴. Most of the usefulmodifications to the siRNA molecules can be introduced using chemistriesestablished for antisense oligonucleotide technology. Preferably, themodifications involve minimal 2′-O-methyl modification, preferablyexcluding such modification. Modifications also preferably excludemodifications of the free 5′-hydroxyl groups of the siRNA.

Synthetic siRNA molecules, including shRNA molecules, can be obtainedusing a number of techniques known to those of skill in the art. Forexample, the siRNA molecule can be chemically synthesized orrecombinantly produced using methods known in the art, such as usingappropriately protected ribonucleoside phosphoramidites and aconventional DNA/RNA synthesizer (see, e.g., ⁷⁵⁻⁷⁹). Alternatively,several commercial RNA synthesis suppliers are available including, butnot limited to, Proligo (Hamburg, Germany), Dharmacon Research(Lafayette, Colo., USA), Pierce Chemical (part of Perbio Science,Rockford, Ill., USA), Glen Research (Sterling, Va., USA), ChemGenes(Ashland, Mass., USA), and Cruachem (Glasgow, UK). As such, siRNAmolecules are not overly difficult to synthesize and are readilyprovided in a quality suitable for RNAi. In addition, dsRNAs can beexpressed as stem loop structures encoded by plasmid vectors,retroviruses and lentiviruses⁸⁰⁻⁸⁷. These vectors generally have apolIII promoter upstream of the dsRNA and can express sense andantisense RNA strands separately and/or as a hairpin structures. Withincells, Dicer processes the short hairpin RNA (shRNA) into effectivesiRNA.

The targeted region of the siRNA molecule of the present invention canbe selected from a given target gene sequence. Nucleotide sequences maycontain 5′ or 3′ UTRs and regions near the start codon. One method ofdesigning a siRNA molecule of the present invention involves identifyingthe 23 nucleotide sequence motif AA(N19)TT (where N can be anynucleotide) and selecting hits with at least 25%, 30%, 35%, 40%, 45%,50%, 55%, 60%, 65%, 70% or 75% G/C content. The “TT” portion of thesequence is optional. Alternatively, if no such sequence is found, thesearch may be extended using the motif NA(N21), where N can be anynucleotide. In this situation, the 3′ end of the sense siRNA may beconverted to TT to allow for the generation of a symmetric duplex withrespect to the sequence composition of the sense and antisense 3′overhangs. The antisense siRNA molecule may then be synthesized as thecomplement to nucleotide positions 1 to 21 of the 23 nucleotide sequencemotif. The use of symmetric 3′ TT overhangs may be advantageous toensure that the small interfering ribonucleoprotein particles (siRNPs)are formed with approximately equal ratios of sense and antisense targetRNA-cleaving siRNPs^(75,76). Analysis of sequence databases, includingbut not limited to the NCBI, BLAST, Derwent and GenSeq as well ascommercially available oligosynthesis companies such as Oligoengine®,may also be used to select siRNA sequences against EST libraries toensure that only one gene is targeted.

Switches

The nucleic acid encoding a gene sensor can be further equipped with aswitch as described previously by Gardner et al, 2000⁴⁷ (incorporatedherein by reference), that allows for generation of athreshold-dependent output rather than a continuous set of outputvalues. A switch may consist of a repressor, which inhibits theexpression of the sensor construct until an agent is applied to permitexpression. The incorporation of switches can tightly regulate the genesensor to limit leakiness and unintended cell death, allowing for safertreatment modalities in a host. Further support is provided byKobayashi, H. et al (2004) PNAS 101(22): 8414-8419, which isincorporated herein by reference⁹⁶.

The nucleic acid encoding the gene sensor is equipped at least with apromoter that is active in the target cell, however the choice ofpromoter can be tailored to target a specific cell population orintracellular condition (e.g., presence of an agent). For example, thegene sensor can be expressed highly in muscle tissue by utilizing themyosin heavy chain promoter on the gene sensor. Various other promotersare known by those skilled in the art.

Target Cell Types

The determination of target cell types is widely variable and can bealtered to meet the specific user's needs. Indeed, the target cell typecan range from various bacterial organisms to various eukaryotic cells.In one embodiment, bacterial cells that are associated with antibioticresistance are utilized and include (but are not limited to)methicillin-resistant Staphylococcus Aureus (MRSA), vancomycin-resistantStaphylococcus Aureus (VSA), Clostridium difficile, Streptococcuspneumoniae, vancomycin-resistant Enterococcus faecalis, Escherichiacoli, Neisseria gonorrhoeae, and Pseudomonas aeruginosa.

In another embodiment, cells involved are eukaryotic or mammalian inorigin. In this context, the cell can be of any cell type including, butnot limited to, epithelial, endothelial, neuronal, adipose, cardiac,skeletal muscle, fibroblast, immune cells, hepatic, splenic, lung,circulating blood cells, reproductive cells, gastrointestinal, renal,bone marrow, and pancreatic cells. The cell can be a cell line, a stemcell, or a primary cell isolated from any tissue including, but notlimited to brain, liver, lung, gut, stomach, fat, muscle, testes,uterus, ovary, skin, endocrine organ and bone, etc.

Introduction of the Construct to a Cell

The nucleic acid molecule or molecules described herein can beintroduced to the cell by any of a number of possible ways. In oneembodiment, the cell is a bacterial cell, which allows the use ofbacteriophage delivery systems to introduce the nucleic acidconstruct(s) to the cell. Bacteriophage therapy is becoming accepted inindustrial and biotechnological settings. For example, the FDA recentlyapproved the use of phage targeted at Listeria monocytogenes as a foodadditive⁹⁹. Phage therapies have also been used successfully in EasternEurope for over 60 years^(100,101) and accordingly the methods of thepresent invention are applicable to human treatment. Even if notdirectly administered to humans or animals, bacteriophage provide adelivery vehicle for administering gene sensor and effector genes tobacteria, for example in a nosocomial setting or in a target populationof bacteria outside of a human host (e.g., biofilm) (see¹⁰²⁻¹⁰⁴, whichare incorporated herein by reference)

The bacteriophage engineered to deliver sensor and/or inhibitor nucleicacid construct as described herein can be a lytic bacteriophage orlysogenic bacteriophage, or any bacteriophage that infects E. coli, P.aeriginosa, S. aureaus, E. facalis and the like and permit the additionof non-bacteriophage sequences. Such phages are well known to oneskilled in the art, and include, but are not limited to, lambda phages,T7, T3, and T-even and T-even like phages, such as T2, and T4, and RB69;also phages such as Pf1, Pf4, Bacteroides fragilis phage B40-8 andcoliphage MS-2 can be used. For example, lambda phage attacks E. coli byattaching itself to the outside of the bacterium and injecting its DNAinto the bacterium. Once injected into the bacterial host, the phageuses E. coli's genetic machinery to transcribe its genes. Any of theknown phage vector systems can be engineered to express the gene sensorsystem described herein. Similarly, where a target species is infectedby another bacteriophage, such bacteriophage can be adapted to use as avector for the introduction of sensor and inhibitor constructs asdescribed herein.

Some phages have been engineered to be more efficient cloning vectors ornaturally lack a gene important in infecting all bacteria, such as maleand female bacteria. These phages would also be suitable for developingthe engineered phages of the present invention. Engineered bacteriophageare described in, for example, U.S. Pat. Nos. 5,258,499 and 6,573,101,which are incorporated herein by reference in their entirety ^(90,91).These bacteriophage were particularly suited to the attack of biofilms,e.g., including biofilms present in human organs, such as colon orlungs.

In another embodiment, the nucleic acid construct is introduced to thecell with a liposome molecule. Liposomes are the first nanotechnology tofind use in a clinical setting. Liposomes are nano sized (50 nm-100 nm)artificial vesicles that are important new materials in the area ofbiotechnology, especially drug and nucleic acid delivery. Phospholipidliposomes have been used in the encapsulation of pharmaceuticals orsupplements for delivery. The liposomic structure provides drugs ashield from external derogatory factors and can deliver the encapsulatedpharmaceutical to cells via endocytosis. Recent advances innanotechnology and molecular biology have enabled the creation of smartliposomes which can target specific cells/organs and which haveprogrammed release of contents (nucleic acids, drugs etc) based onexternal or internal cues, e.g., pH, temperature or the presence of acomplementary enzyme, among others. Use of liposome technology in termsof the method described herein relates to the encapsulation, protectionfrom degradation and delivery of the nucleic acid construct orconstructs to cells. Liposomes can be engineered such that the moleculesare specifically targeted to a particular cell type by recognition of areceptor on the external surface of the cell (as previously disclosed,for example, in U.S. Pat. Nos. 5,258,499 and 6,573,101, which areincorporated herein by reference^(92,93)). Liposomal delivery mechanismscomprising the nucleic acid constructs and variants described hereinallow a wide variety of selectivity to treat specific disease states byselectively targeting liposomes to cells based on the surface expressionof cellular receptors. For example, liposomes can be targeted tocancerous cells that express tumor antigens on the plasma membrane,thereby mediating the delivery of a gene sensor system as describedherein to cells involved in tumor progression. Of course, part of thestrength of the methods described herein is related to the restrictionof effector gene expression to cells expressing a target gene. Thus,while liposomes or other delivery approaches can be tailored to targetspecific cells, even if such delivery is not 100% restricted to thosecells, it is anticipated that the effector gene expression will besubstantially limited to those cells expressed in the target gene.

In another embodiment, the nucleic acid molecule is introduced tobacterial cells in culture by bacterial transformation methods such ascalcium chloride mediated transformation or other treatment renderingcells competent to take up exogenous nucleic acids. In anotherembodiment, the nucleic acid molecule is introduced to mammalian cellsby the use of a viral delivery system (e.g., adenovirus, lentivirus).Both adenoviral and lentiviral vectors are capable of infecting dividingand non-dividing mammalian cells. Adenoviral vectors are often employedby those skilled in the art to produce a transient, high expression of anucleic acid construct in question. Adenoviruses containing a nucleicacid construct are recognized by coxsackie receptors on the surface ofmammalian cells, and subsequently enter the cell via endocytosis. Thenucleic acid construct of adenoviral vectors is retained episomally,which does not permit cells to pass the nucleic acid constructinformation to daughter cells by genetic means. Alternatively, nucleicacid constructs delivered by a lentiviral system are incorporated intothe host genome and thus are often employed for longer term expression,although at a lower expression level than that observed with adenoviralvectors.

In another embodiment, the nucleic acid molecule is introduced to cellsin a human host in a conjugated form. The nucleic acid molecule can beconjugated, for example, to a cholesterol molecule, which may allowincorporation of the nucleic acid molecule into a circulatinglipoprotein that would result in reduced clearance rates of theintroduced construct. Alternatively, the nucleic acid molecule can beconjugated to an aptamer molecule (e.g., an oligonucleotide orpolypeptide sequence), which can be targeted to a specific population ofcells by altering the aptamer sequence to allow recognition by certaincell types. Similarly, the nucleic acid molecule can be conjugated to anantibody, which also allows cell specific recognition and uptake of thenucleic acid molecule. Additionally, the nucleic acid molecule can beconjugated to a cationic polypeptide and polymer mix to form stablenanoparticles, which can be incorporated with polyethylene glycol (PEG)to prevent aggregation of the particles.

In another embodiment, the nucleic acid molecule is introduced tolocalized tissues in vivo by direct localized injection with or withoutelectroporation. The nucleic acid can be injected in e.g., anintra-ocular, intracerebroventricular, intrathecal, or intraparenchymalmanner. In another embodiment, the nucleic acid is locally introduced tothe airways by inhalation, intranasal or orotracheal administration.Alternatively, the nucleic acid molecule is delivered by a liposomedesigned to deliver the gene sensor to the target tissue by injectioninto the systemic circulation. Dosages of construct delivery agents willnecessarily vary with respect to the target cell and the organismcontaining the target cell. Thus, for example, where the target cell iscompared by a higher eukaryote, e.g., a mammal, the dose could beadapted in that animal, or will act as the specific delivery agent anddelivery avenue. Thus, localized injection, for example, would require alower dose than, for example, systemic injection. Similarly, a targeteddelivery approach would be expected to require a lower dose than anon-targeted delivery approach. Where, for example, the constructs areto be introduced to a bacterial cell, the dosage will preferably beadjusted in relation to the size of the target cell population, i.e,higher doses, such as higher multiplicity of infection (MOI) wherebacteriophage are concerned, can be used when the bacterial cellpopulation is larger, relative to another population. MOI can beadjusted by the skilled artisan as necessary to achieve the desiredlevel of delivery.

In another embodiment, bacteria can be transformed with the nucleic acidencoding the gene sensor such that the components are expressed in thebacterial cells. The transformed bacteria can then be used to deliverthe components into mammalian cells as has been previously described byXiang et el⁶⁸.

Dosing and Efficacy

Those skilled in the art of administration and dose regimes candetermine the appropriate dose required for disease management using thegene sensor technology described herein. The efficacy of the gene sensortechnology can be determined by a reduction in symptoms, for exampleskin lesions from infection with Staphylococcus Aureus. Alternatively,the efficacy can be measured by a reduction in the severity of diseaseas determined by one skilled in the art of diagnosis based on ameasurable disease severity grading scale such as, for example the NYHAClasses of Heart failure. In this example, there are four stages ofheart failure graded from mild to severe, based on symptoms such ase.g., the ability to carry on physical activity, shortness of breath,and palpitations. Efficacy can be measured in this example by themovement of a patient from e.g., a Class IV (severe) heart failureprofile to a Class III, Class II, or Class I heart failure profile.Similar grading scales exist, for example, for heart disease, diabeticretinopathy, systemic sclerosis, Clostridium difficile—AssociatedDisease, Lipodystrophy Severity Grading Scale, HIV Outpatient Studyscale, among others, which can be used to determine a patients' progressin response to treatment. Any positive change resulting in e.g.,lessening of severity of disease measured using the appropriate scale,represents adequate treatment using the gene sensor described herein.

A treatment is considered “effective treatment,” as the term is usedherein, if any one or all of the signs or symptoms of e.g., a biofilmassociated disease or cancer are altered in a beneficial manner or otherclinically accepted symptoms or markers of disease are improved, or evenameliorated, e.g., by at least 10% following treatment involving a genesensor as described herein. Efficacy can also be measured by a failureof an individual to worsen as assessed by hospitalization or need formedical interventions (i.e., progression of the disease is halted or atleast slowed). Methods of measuring these indicators are known to thoseof skill in the art and/or described herein. Treatment includes anytreatment of a disease in an individual or an animal (some non-limitingexamples include a human, or a mammal) and includes: (1) inhibiting thedisease, e.g., arresting, or slowing the pathogenic growth of e.g., abacterium, a virus, or a fungus; or (2) relieving the disease, e.g.,causing regression of symptoms, reducing the level of infection; and (3)preventing or reducing the likelihood of the development of infectione.g., in an immunocompromised individual.

Determining Reference Level and Measuring Expression

In the methods of target gene detection described herein, the expressionof the effector gene sequence is preferably monitored by comparing thelevel of effector gene expression to a reference level.

To determine the reference level, the sensor and inhibitor construct(s)is/are delivered to a cell that does not express the target gene(referred to herein as a “control” or “reference” cell). Confirmationthat a cell lacks target gene expression is performed, for example, atthe mRNA level utilizing quantitative Real-Time PCR with target genespecific primers or Northern blot analysis with target gene specificprobes. Alternatively, when the target gene is, e.g., a resistance gene,the lack of the target gene in a reference population or cell can beconfirmed by treatment of cells with the agent to which the target geneconfirms resistance. Once a reference cell has been established to lackthe target gene, the sensor and inhibitor nucleic acid construct(s)is/are introduced to the cells by a delivery mechanism as known in theart of as described herein (e.g., bacteriophage, liposomes, viralcarriers). It is preferred that the effector gene expression isdetermined by fluorescence of a reporter molecule (e.g., FACS), howevereffector gene expression can also be determined by PCR, Northern blotanalysis or Western blot analysis. Where the effector is a reporter, thenormal avenue of reporter detection is followed. The level of effectorexpression in cells lacking target gene expression denotes the referencelevel of the system.

To measure or titrate the expression of an effector molecule, thenucleic acid construct is delivered to a population of cells known toexpress the target gene of interest. Analysis of reporter geneexpression can be determined, for example, by sorting cells using afluorescence assisted cell sorter (FACS) machine according to the levelof fluorescence detected when the reporter is a fluorescent molecule.The intensity of fluorescence in cells that actively express the targetgene is normalized to the intensity of fluorescence in cells that do notactively express the target gene. The amounts of inhibitory nucleic acidconstruct or expression, or the amounts of sensor construct orexpression, can be fine tuned with control or reference populationsknown to express or not express the target gene, as the case may be.Amounts of inhibitory nucleic acid can be adjusted, for example, byincreasing or decreasing the amount of construct in produced strength ofpromoter from which it is expressed. Similar adjustments can be appliedto the sensor construct.

Mediating Inhibition

In order to determine the portion of the target sequence that issufficient to mediate inhibition by an inhibitory nucleic acid sequenceas described herein, the sensor construct and the inhibitory nucleicacid construct can first be encoded on two separate nucleic acidmolecules (e.g., vectors). In cells previously determined to express thetarget gene, the sensor construct is preferably expressed at a maximallevel of reporter expression, such that a further increase in sensorconstruct delivery does not result in an increase in reporterexpression. The nucleic acid molecule encoding the inhibitory nucleicacid molecule is then delivered in varying amounts to titrate the amountof expression. Any resulting changes in reporter expression as a resultof altering the dose of inhibitory nucleic acid is analyzed and theinhibition level assessed. If there is no inhibition or very littleinhibition of reporter fluorescence with a reasonable dose of inhibitorynucleic acid, then the portion of the target sequence coupled to theeffector is not sufficient to mediate inhibition by binding to theinhibitory nucleic acid sequences. The length of the target gene portionof the sensor construct can be altered to modify the strength of bindingto the inhibitory nucleic acid sequence. Generally, nucleic acidsequences bind in a strong manner when the sequences have a low degreeof mis-match, and/or contain a high G-C content. These characteristicscan be considered in the design of the sensor construct.

It is also contemplated herein that the threshold of the sensor isadjusted by modifying the antisense RNA sensor to bind more or lesstightly to the target or the effector. The amount of acceptableselective sensing will depend on the application and can be determinedby one of skill in the art.

Gene Sensor Detection Kit

Disclosed herein is a gene sensor detection kit comprising the genesensor described herein and packaging materials therefor. The kit willcontain nucleic acids encoding a sensor construct and an inhibitorynucleic acid vector, reagents, and instructions for proper use. As inother aspects described herein, the sensor construct and inhibitorynucleic acid construct can be present on separate vectors, or,preferably, on the same vector.

In one aspect, the kits described herein contain one or more control orreference cells, e.g., a positive control that expresses a target gene,and/or a negative control that does not express the target gene. Thekits can further comprise reagents necessary for the delivery of theconstruct(s) to cells, e.g., liposomes or other delivery reagents, aswell as, for example, antibiotics or other drugs suitable to test thesystem in a given setting.

In one aspect, a kit is adapted to be used on in vitro samples, e.g.,taken from a patient suspected of harboring a resistant cell. Samplescan comprise, for example, blood, serum, urine, sputum, a tissue sample,or any other biological sample derived from a subject suspected toharbor a target organism.

The methods and compositions described herein lend themselves to anumber of different applications, of varying scope. Further aspects ofthe methods, compositions and kits described herein include at least thefollowing.

Further Embodiments

It can be advantageous to use the gene sensor approaches describedherein to detect not just individual genes, such as resistance genes,but also to detect broader genera of targets. For example, in oneaspect, the target gene is characteristic of a particular group oforganisms, e.g., bacteria, fungi or viruses. In such aspects, a panel ofgene sensor targets can be employed to quickly determine whether asample contains, for example, a bacterium, a virus, or a fungus. Thisapproach can provide reagents to narrow the possibilities for an agentof disease or type of contamination present. Possibilities for thedetection of such genera include, for example, detection of bacterialribosomal RNA or other gene targets characteristic of bacteria. Virusescan be detected by targeting, for example, viral polymerase, viralintegrase, viral thymidine kinase, or other genes characteristic ofviruses. Fungi can be targeted in a similar manner using a gene targetor targets on a panel that are characteristic of fungi. This approachcan be modified to include, rather than a single target that ischaracteristic of a given genera, e.g., bacteria in general, a panel oftargets that would be diagnostic for any of a number of differentmembers of the genus, e.g., gram negative bacteria, gram positivebacteria, or particular groups of bacteria, e.g., cocci, mycobacteria,spirochetes, etc. If desired, any such panel can be further segmented,or sub-panels provided, e.g., to provide markers specific for particularsubgroups, up to and including specific species, e.g., Staphylococcusaureus, or even methicillin-resistant Staphylococcus aureus, as but afew examples. In this manner, the methods described herein can be usedto prepare panels of varying degrees of specificity for different usesin a clinical, veterinary, agricultural or industrial setting. Kitscontaining such panels and reagents necessary to use them arespecifically contemplated herein.

A multiplex format can also be used for the methods described herein.That is, gene sensor circuit reagents (including, at a minimum, a genesensor construct and an inhibitory nucleic acid construct specific forthe target of the sensor) for a plurality (i.e., at least two, at leastthree, at least four, at least five or more) of targets can be includedin kits for the introduction to a desired cell type. In this aspect,each effector for each different target provides signal or read-out(i.e., reporter activity) that is distinguishable from that of each ofthe other effectors. As with other aspects described herein, theconstructs making up the individual sensor circuits can be integratedinto one vector or into separate vectors designed to function in thesame cell type. In this manner, the circuits described herein can beused to determine which of a plurality of targets is present in a givenpopulation of cells. In this aspect, as in others described herein,effectors can include, as but a few examples, fluorescent proteins withdistinguishable emission spectra, or enzymes that generate differentcolorimetric read-outs with appropriate substrates.

Finally, the embodiments described herein generally focus on a situationin which effector expression is maintained in an “off” position unless atarget gene is expressed. However, circuits in which effector expressionis in the “on” position as a default in the absence of target geneexpression, and becomes down-regulated when the target gene is expressedare specifically contemplated.

It is understood that the foregoing detailed description and thefollowing examples are illustrative only and are not to be taken aslimitations upon the scope of the invention. Various changes andmodifications to the disclosed embodiments, which will be apparent tothose skilled in the art, may be made without departing from the spiritand scope of the present invention. Further, All patents, patentapplications, and publications identified are expressly incorporatedherein by reference for the purpose of describing and disclosing, forexample, the methodologies described in such publications that might beused in connection with the present invention. These publications areprovided solely for their disclosure prior to the filing date of thepresent application. Nothing in this regard should be construed as anadmission that the inventors are not entitled to antedate suchdisclosure by virtue of prior invention or for any other reason. Allstatements as to the date or representation as to the contents of thesedocuments is based on the information available to the applicants anddoes not constitute any admission as to the correctness of the dates orcontents of these documents.

The present invention may be as defined in any one of the followingnumbered paragraphs:

-   1. A method of detecting a cell expressing a target gene comprising    the steps of:    -   a. introducing to the cell a nucleic acid construct encoding a        portion of the target gene sequence, fused to an effector gene        sequence, and a construct encoding an inhibitory nucleic acid        sequence directed against at least the portion of the target        gene sequence, wherein in the absence of the target gene, the        inhibitory nucleic acid sequence suppresses effector gene        expression; and    -   b. monitoring expression of the effector gene sequence in the        cell and comparing the level of effector gene expression to a        reference level, wherein if expression of the effector gene is        increased relative to the reference level, the target gene is        detected in the cell.-   2. The method of paragraph 1, wherein the target gene is a mutant    gene.-   3. The method of paragraph 1 or 2, wherein the nucleic acid    construct encoding a portion of the target gene sequence, fused to a    the effector gene sequence, and the construct encoding an inhibitory    nucleic acid sequence are encoded on the same nucleic acid molecule.-   4. The method of paragraph 1, 2, or 3, wherein the cell is a    bacterial cell.-   5. The method of paragraph 1, 2 or 3, wherein the cell is a    eukaryotic cell.-   6. The method of any one of paragraphs 1-4, wherein the target gene    is a drug resistance or virulence gene.-   7. The method of any one of paragraphs 1-6, wherein the target gene    is a multi-drug resistance (MDR) transporter gene.-   8. The method of any one of paragraphs 1-7, wherein the target gene    is a viral gene.-   9. The method of any one of paragraphs 1-8, wherein the target gene    is an oncogene or tumor associated gene.-   10. The method of any one of paragraphs 1-9, wherein the effector    gene sequence encodes a reporter molecule.-   11. The method of any one of paragraphs 1-10, wherein the reporter    comprises a polypeptide selected from the group consisting of an    enzyme, a fluorescent polypeptide, a luminescent polypeptide and an    antigen.-   12. An engineered gene sensor comprising a nucleic acid sequence    encoding a portion of a target gene sequence fused to an effector    gene sequence and further comprising a construct encoding an    inhibitory nucleic acid sequence directed against at least the    portion of the target gene sequence, wherein the presence of the    target gene expression in a cell permits the effector gene    expression to produce an effector polypeptide which mediates    delivery of an agent or mediates cell killing.-   13. The gene sensor of paragraph 12, wherein the nucleic acid    construct encoding a portion of the target gene sequence, fused to a    the effector gene sequence, and the construct encoding an inhibitory    nucleic acid sequence are encoded on the same nucleic acid molecule.-   14. The gene sensor of paragraph 12 or 13, wherein the nucleic acid    molecule comprises a vector.-   15. The gene sensor of paragraph 12, 13, or 14, wherein the target    gene is a drug resistance or virulence gene.-   16. The gene sensor of any one of paragraphs 12-15, wherein the    target gene is a multi-drug resistance (MDR) transporter gene.-   17. The gene sensor of any one of paragraphs 12-16, wherein the    target gene is a viral gene.-   18. The gene sensor of any one of paragraphs 12-17, wherein the    target gene is an oncogene or tumor associated gene.-   19. The gene sensor of any one of paragraphs 12-18, wherein the    effector gene sequence encodes a reporter molecule.-   20. The gene sensor of any one of paragraphs 12-19, wherein the    reporter comprises a polypeptide selected from the group consisting    of an enzyme, a fluorescent polypeptide, a luminescent polypeptide    and an antigen.-   21. A method of selectively killing a cell expressing a target gene,    the method comprising the steps of introducing to the cell a nucleic    acid construct encoding a portion of the target gene sequence, fused    to an effector gene sequence, and a construct encoding an inhibitory    nucleic acid sequence directed against at least the portion of the    target gene sequence,    -   wherein in the absence of target gene expression, the inhibitory        nucleic acid sequence suppresses expression of the effector        gene, and    -   wherein the presence of target gene expression in the cell        permits effector gene expression to produce an effector        polypeptide, and wherein expression of the effector polypeptide        mediates cell killing.-   22. The method of paragraph 21, wherein the cell is a bacterial    cell.-   23. The method of paragraph 21 or 22, wherein the cell is a    eukaryotic/mammalian cell.-   24. The method of paragraph 21, 22, or 23, wherein the target gene    is a drug resistance or virulence gene.-   25. The method of any one of paragraphs 21-24, wherein the target    gene is a mutant gene.-   26. The method of any one of paragraphs 21-25, wherein the target    gene is a MDR transporter gene.-   27. The method of any one of paragraphs 21-26, wherein the target    gene is an oncogene.-   28. The method of any one of paragraphs 21-27, wherein the target    gene is a viral gene.-   29. The method of any one of paragraphs 21-28, wherein the effector    gene sequence encodes a polypeptide that mediates cell killing.-   30. The method of any one of paragraphs 21-29, wherein the effector    gene sequence encodes an RNA interference molecule.-   31. The method of any one of paragraphs 21-30, wherein the effector    is an apoptotic gene.-   32. The method of any one of paragraphs 21-31, wherein the effector    inhibits angiogenic pathways.-   33. A system for detecting a target gene, the system comprising:    -   a. a nucleic acid construct encoding a portion of the target        gene sequence, fused to an effector gene sequence; and    -   b. a construct encoding an inhibitory nucleic acid sequence that        inhibits the target gene expression, wherein the nucleic acid        constructs of (a) and (b) are comprised by the same nucleic acid        molecule.-   34. The system of paragraph 33, wherein the nucleic acid molecule    comprises a vector.-   35. The system of paragraph 33 or 34, wherein the vector is selected    from the group consisting of: a plasmid vector, a viral vector and a    bacteriophage vector.-   36. The system of paragraph 33, 34 or 35, wherein the effector gene    sequence encodes a reporter.-   37. The system of paragraph 36, wherein the reporter comprises a    polypeptide selected from the group consisting of an enzyme, a    fluorescent polypeptide, a luminescent polypeptide and an antigen.-   38. The system of any one of paragraphs 33-37, wherein the effector    gene sequence encodes a polypeptide that mediates cell killing.-   39. The system of any one of paragraphs 33-38, wherein the effector    gene sequence encodes an RNA interference molecule.-   40. The system of any one of paragraphs 33-39, wherein the target    gene sequence is comprised by a drug resistance gene or a virulence    gene.-   41. The system of any one of paragraphs 33-40, wherein the target    gene is an MDR transporter gene.-   42. The system of any one of paragraphs 33-41, wherein the target    gene sequence is comprised by a viral gene.-   43. The system of any one of paragraphs 33-42, wherein the target    gene sequence is comprised by a target oncogene or tumor associated    gene.-   44. The system of any one of paragraphs 33-43, wherein the effector    is an apoptotic gene.-   45. The system of any one of paragraphs 33-44, wherein the effector    inhibits angiogenic pathways.-   46. A kit for detecting a target gene, the system comprising:    -   a. a nucleic acid construct encoding a portion of the target        gene sequence, fused to an effector gene sequence;    -   b. a construct encoding an inhibitory nucleic acid sequence that        inhibits the target gene expression, wherein the nucleic acid        constructs of (a) and (b) are comprised by the same nucleic acid        molecule; and    -   c. packaging materials therefore.-   47. The kit of paragraph 46, wherein the nucleic acid construct    encoding a portion of the target gene sequence, fused to a the    effector gene sequence, and the construct encoding an inhibitory    nucleic acid sequence are encoded on the same nucleic acid molecule.-   48. The kit of paragraph 46 or 47, wherein the nucleic acid molecule    comprises a vector.-   49. The kit of paragraph 46, 47, or 48, wherein the target gene is a    drug resistance or virulence gene.-   50. The kit of any one of paragraphs 46-49, wherein the target gene    is a multi-drug resistance (MDR) transporter gene.-   51. The kit of any one of paragraphs 46-49, wherein the target gene    is a viral gene.-   52. The kit of any one of paragraphs 46-51, wherein the target gene    is an oncogene or tumor associated gene.-   53. The kit of any one of paragraphs 46-52, wherein the effector    gene sequence encodes a reporter molecule.-   54. The kit of any one of paragraphs 46-53, wherein the reporter    comprises a polypeptide selected from the group consisting of an    enzyme, a fluorescent polypeptide, a luminescent polypeptide and an    antigen.-   55. Use of a nucleic acid construct encoding a portion of a target    gene sequence, fused to an effector gene sequence, and a construct    encoding an inhibitory nucleic acid sequence directed against at    least a portion of the target gene sequence for selectively killing    a cell, the use comprising introducing the nucleic acid sequence to    a cell.    -   wherein in the absence of the target gene sequence, the        inhibitory nucleic acid sequence suppresses effector gene        expression, and    -   wherein the presence of target gene expression in the cell        permits effector gene expression to produce an effector        polypeptide, and wherein expression of the effector polypeptide        mediates cell killing.-   56. Use of an engineered sensor comprising a nucleic acid sequence    encoding a portion of a target gene sequence fused to an effector    gene sequence and further comprising a construct encoding an    inhibitory nucleic acid sequence directed against at least the    portion of the target gene sequence for selectively killing a cell,    the use comprising administering the engineered sensor to a cell,    -   wherein the presence of the target gene expression in the cell        permits the effector gene expression to produce an effector        polypeptide which mediates delivery of an agent or mediates cell        killing.

EXAMPLES Example 1 A Bacterial Gene Sensor Circuit

As a first demonstration of the gene sensor circuit, described herein,an inhibitory nucleic acid construct was designed to express anantisense RNA target; the antisense resistance gene CAT. This wascoupled with a sensor-effector construct with a portion of the CAT genefused to GFP (odi) sequences. The read-out is GFP fluorescence.

In one example, the paired-termini (PT7) design described by Nakashimaet al, 2006⁴⁶ is extended to produce an antisense RNA similar to thatshown in FIG. 1. The PT7 construct produces an antisense RNA with longerhalf-lives in vivo, allowing for greater antisense effect⁴⁶. Startingwith the PT7 construct shown in FIG. 1, the NcoI and XhoI restrictionenzyme sites were replaced by HindIII and NheI sites, respectively. Inthis example, the target gene encodes for chloramphenicol resistance andis denoted as cat. This target gene represents a very well-characterizedand important gene in E. coli encoding chloramphenicol resistance⁹⁴.Using the paired-termini (PT) asRNA system⁴⁶, an antisense RNA wasdesigned to target cat mRNA primarily in the 5′-untranslated region, theribosome binding site (RBS) region, and the 5′ end of the coding region(from the cat transcriptional start site to base pair 300 in the catgene)⁴⁶. DNA encoding antisense RNA to the antibiotic resistance genetarget chloramphenicol acetyltransferase (cat, conferringchloramphenicol resistance (Cm^(R))) was cloned in between HindIII andNheI sites near the top of the stem of the PT7 construct. The resultingPT7-cat-asRNA construct produces an antisense RNA that can bind to catRNA and repress cat expression. The PT7-cat-asRNA was cloned under theinducible control of anhydrotetracycline (aTc) by using pZE21Y12α12GFP^(95,105), taking care to remove the cis-repressive sequence and GFP. Inorder to design a plasmid that can detect the presence of target catmRNA, the cat gene was fused with the gfp gene to produce a construct(cat-gfp) that is inhibited by PT7-cat-asRNA and thus produces lowlevels of GFP normally. A portion of cat DNA that encodes the5′-untranslated region, the RBS, and the 5′ end of the coding region wasfused to gfp at a NotI restriction site. In this design, the cat-gfpfusion acts as the effector component (here, the effector is a reporter)while the PT7-cat-asRNA acts as the sensor component as depicted in FIG.2. The cat-gfp fusion was then placed under the control of P_(BAD) inthe same vector as the sensor. The resulting sensor-reporter plasmid(pZE21s1-cat) was cloned into E. coli DH5αPro cells by itself or with aplasmid that either carried cat (pZA3) or bla (pZA1). pZA3 and pZA1 wereboth constructed from plasmids described by Lutz et al, 1997⁹⁵ with thesynthetic promoters and genes removed by deleting all DNA between theXhoI and XmaI restriction sites. The pZE21s1-cat plasmid was introducedinto E. coli DH5αPro cells alone^(95,105), with a plasmid containing bla(pZA1), or with a plasmid containing cat (pZA3). GFP output in thepresence of cat was approximately twice the output obtained in thepresence of bla or without any plasmid at all as depicted in FIG. 4.Thus, pZE21s1-cat is able to detect the presence of a specific mRNAspecies encoding an antibiotic-resistance gene in vivo.

Example 2

In a second example, an RNA sensor design utilizes the pTAK series ofplasmids described by Gardner et al, 2000⁴⁷. Instead of cloning theantisense RNA for the target RNA into the upstream region of lacI, thePT7-cat-asRNA construct is placed downstream of the lacI gene to createpTAKs3-cat as depicted in FIG. 6. This design is utilized for targetgenes where the primary mode of the PT7 construct's antisense RNA actionis through RNA degradation, as this system relies on lacI-PT7-cat-asRNAbinding to cat mRNA to trigger degradation of the lacI-PT7-cat-asRNARNA⁴⁶. Reduction of the levels of the Lac repressor increase GFP output.The RNA sensor design shown in FIG. 5 is realized by cloning PT7-asRNAfragments into the AscI site in pTAK132 from Gardner et al, 2000⁴⁷. ThecI857 gene in pTAK132 is removed by restriction digest in order toobtain a continuous output behavior rather than discontinuous switchbehaviour. This permits the effector molecule to be in two main states:on and off. The sensor-reporter plasmid (pTAKs3-cat) can beco-transformed with cat- or kan-containing plasmids (pZA3 or pZA2,respectively) for testing.

Example 3

The following example demonstrates how to determine the sensitivity andfalse positive rate of the gene sensor described herein. Generally, ahigher false positive rate is preferred, i.e., greater sensitivity, atthe potential expense of some false positives, as opposed to a higherfalse negative rate, in which positive cells are not detected.

Definitions

-   -   True Positive (TP)—cell has target gene and is sensed    -   False Negative (FN)—cell has target gene but is not sensed    -   True Negative (TN)—cell doesn't have target gene and is not        sensed    -   False Positive (FP)—cell doesn't have target gene but is sensed

Using standard statistical terms, let us define selective sensing as alow false-positive rate (meaning that a low number of cells that do nothave the target are sensed or killed=FP/(TN+FP))

Still want respectable sensitivity (meaning that a good number of cellsthat do have the target are actually sensed=TP/(TP+FN))

FIG. 6 shows the sensitivity and false-positive rate on the Y-axisversus different thresholds on the X-axis (thresholds of fluorescenceintensity) for FIG. 4 c in “Gene Sensor Description.doc” (t=6.25 hours)

False-positives calculated for pZE21s1-cat (i.e. the situation wherethere is no target gene in the cells)

REFERENCES

All references cited in the specification and listed below, are hereinincorporated by reference in their entirety.

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TABLE 1 CTCGAGTCCCTATCAGTGATAGAGATTGACATCCCTATCAGTGATAGAGATACTGAGCACATCAGCAGGACGCACTGACCGAATTC AGGAGGAATT AACCATGCAG TGGTGGTGGT GGTGGTG AAGCTTTATAGGTACATTGAGCAACTGACTGAAATGCCTCAAAATGTTCTTTACGATGCCATTGGGATATATCAACGGTGGTATATCCAGTGATTTTTTTCTCCATTTTAGCTTCCTTAGCTCCTGAAAATCTCGATAACTCAAAAAATACGCCCGGTAGTGATC GCTAGC CACCACC ACCACCACCA CTGCATGGTT AATTCCTCCT CCCGGG GGATCCCATGGT ACGCGT GGCATCAAATAAAACGAAAGGCTCAGTCGAAAGACTGGGCCTTTCGTTTTATCTGTTGTTTGTCGGTGAACGCTCTCCTGAGTAGGACAAATCCGCCGCCCTAGA CCTAGG     CGTTCGGCTGCGGCGAGCGGTATCAGCTCACTCAAAGGCGGTAATACGGTTATCCACAGAATCAGGGGATAACGCAGGAAAGAACATGTGAGCAAAAGGCCAGCAAAAGGCCAGGAACCGTAAAAAGGCCGCGTTGCTGGCGTTTTTCCATAGGCTCCGCCCCCCTGACGAGCATCACAAAAATCGACGCTCAAGTCAGAGGTGGCGAAACCCGACAGGACTATAAAGATACCAGGCGTTTCCCCCTGGAAGCTCCCTCGTGCGCTCTCCTGTTCCGACCCTGCCGCTTACCGGATACCTGTCCGCCTTTCTCCCTTCGGGAAGCGTGGCGCTTTCTCAATGCTCACGCTGTAGGTATCTCAGTTCGGTGTAGGTCGTTCGCTCCAAGCTGGGCTGTGTGCACGAACCCCCCGTTCAGCCCGACCGCTGCGCCTTATCCGGTAACTATCGTCTTGAGTCCAACCCGGTAAGACACGACTTATCGCCACTGGCAGCAGCCACTGGTAACAGGATTAGCAGAGCGAGGTATGTAGGCGGTGCTACAGAGTTCTTGAAGTGGTGGCCTAACTACGGCTACACTAGAAGGACAGTATTTGGTATCTGCGCTCTGCTGAAGCCAGTTACCTTCGGAAAAAGAGTTGGTAGCTCTTGATCCGGCAAACAAACCACCGCTGGTAGCGGTGGTTTTTTTGTTTGCAAGCAGCAGATTACGCGCAGAAAAAAAGGATCTCAAGAAGATCCTTTGATCTTTTCTACGGGGTCTGACGCTCAGTGGAACGAAAACTCACGTTAAGGGATTTT GGTCATGACTAGT GCTTGGATTCTCACCAATAAAAAACGCCCGGCGGCAACCGAGCGTTCTGAACAAATCCAGATGGAGTTCTGAGGTCATTACTGGATCTATCAACAGGAGTCCAAGC GAGCTCTCGAACCCCAGAGTCCCGCTCAGAAGAACTCGTCAAGAAGGCGATAGAAGGCGATGCGCTGCGAATCGGGAGCGGCGATACCGTAAAGCACGAGGAAGCGGTCAGCCCATTCGCCGCCAAGCTCTTCAGCAATATCACGGGTAGCCAACGCTATGTCCTGATAGCGGTCCGCCACACCCAGCCGGCCACAGTCGATGAATCCAGAAAAGCGGCCATTTTCCACCATGATATTCGGCAAGCAGGCATCGCCATGGGTCACGACGAGATCCTCGCCGTCGGGCATGCGCGCCTTGAGCCTGGCGAACAGTTCGGCTGGCGCGAGCCCCTGATGCTCTTCGTCCAGATCATCCTGATCGACAAGACCGGCTTCCATCCGAGTACGTGCTCGCTCGATGCGATGTTTCGCTTGGTGGTCGAATGGGCAGGTAGCCGGATCAAGCGTATGCAGCCGCCGCATTGCATCAGCCATGATGGATACTTTCTCGGCAGGAGCAAGGTGAGATGACAGGAGATCCTGCCCCGGCACTTCGCCCAATAGCAGCCAGTCCCTTCCCGCTTCAGTGACAACGTCGAGCACAGCTGCGCAAGGAACGCCCGTCGTGGCCAGCCACGATAGCCGCGCTGCCTCGTCCTGCAGTTCATTCAGGGCACCGGACAGGTCGGTCTTGACAAAAAGAACCGGGCGCCCCTGCGCTGACAGCCGGAACACGGCGGCATCAGAGCAGCCGATTGTCTGTTGTGCCCAGTCATAGCCGAATAGCCTCTCCACCCAAGCGGCCGGAGAACCTGCGTGCAATCCATCTTGTTCAATCATGCGAAACGATCCTCATCCTGTCTCTTGATCAGATCTTGATCCCCTGCGCCATCAGATCCTTGGCGGCAAGAAAGCCATCCAGTTTACTTTGCAGGGCTTCCCAACCTTACCAGAGGGCGCCCCAGCTGGCAATTCC GACGTC TTT GGGCCCAAGGCCCAGTCTTTCGACTGAGCCTTTCGTTTTATTTGATGCCTGGCAGTTCCCTACTCTCGCATGGGGAGACCCCACACTACCATCGGCGCTACGGCGTTTCACTTCTGAGTTCGGCATGGGGTCAGGTGGGACCACCGCGCTACTGCCGCCAGGCAAATTCTGTT TCTAGATTATTTGTATAGTTCATCCATGCCATGTGTAATCCCAGCAGCTGTTACAAACTCAAGAAGGACCATGTGGTCTCTCTTTTCGTTGGGATCTTTCGAAAGGGCAGATTGTGTGGACAGGTAATGGTTGTCTGGTAAAAGGACAGGGCCATCGCCAATTGGAGTATTTTGTTGATAATGGTCTGCTAGTTGAACGCTTCCATCTTCAATGTTGTGTCTAATTTTGAAGTTAACTTTGATTCCATTCTTTTGTTTGTCTGCCATGATGTATACATTGTGTGAGTTATAGTTGTATTCCAATTTGTGTCCAAGAATGTTTCCATCTTCTTTAAAATCAATACCTTTTAACTCGATTCTATTAACAAGGGTATCACCTTCAAACTTGACTTCAGCACGTGTCTTGTAGTTCCCGTCATCTTTGAAAAATATAGTTCTTTCCTGTACATAACCTTCGGGCATGGCACTCTTGAAAAAGTCATGCTGTTTCATATGATCTGGGTATCTCGCAAAGCATTGAACACCATAACCGAAAGTAGTGACAAGTGTTGGCCATGGAACAGGTAGTTTTCCAGTAGTGCAAATAAATTTAAGGGTAAGTTTTCCGTATGTTGCATCACCTTCACCCTCTCCACTGACAGAAAATTTGTGCCCATTAACATCACCATCTAATTCAACAAGAATTGGGACAACTCCAGTGAAAAGTTCTTCTCCTTTACG C GCGGCCGCcgccccgccctgccactcatcgcagtactgttgtaattcattaagcattctgccgacatggaagccatcacagacggcatgatgaacctgaatcgccagcggcatcagcaccttgtcgccttgcgtataatatttgcccatggtgaaaacgggggcgaagaagttgtccatattggccacgtttaaatcaaaactggtgaaactcacccagggattggctgagacgaaaaacatattctcaataaaccctttagggaaataggccaggttttcaccgtaacacgccacatcttgcgaatatatgtgtagaaactgccggaaatcgtcgtggtattcactccagagcgatgaaaacgtttcagtttgctcatggaaaacggtgtaacaagggtgaacactatcccatatcaccagctcaccgtctttcattgccatacggaattccggatgagcattcatcaggcgggcaagaatgtgaataaaggccggataaaacttgtgcttatttttctttacggtctttaaaaaggccgtaatatccagctgaacggtctggttataggtacattgagcaactgactgaaatgcctcaaaatgttctttacgatgccattgggatatatcaacggtggtatatccagtgatttttttctccattttagcttccttagctcctgaaaatctcgataactcaaaaaatacgcccggtagtgatc GTCGACTATGGAGAAACAGTAGAGAGTTGCGATAAAAAGCGTCAGGTAGGATCCGCTAATCTTATGGATAAAAATGCTATGGCATAGCAAAGTGTGACGCCGTGCAAATAATCAATGTGGACTTTTCTGCCGTGATTATAGACACTTTTGTTACGCGTTTTTGTCATGGCTTTGGTCCCGCTTTGTTACAGAATGCTTTTAATAAGCGGGGTTACCGGTTTGGTTAGCGAGAAGAGCCAGTAAAAGACGCAGTGACGGCAATGTCTGATGCAATATGGACAATTGGTTTCTT

TABLE 2 Signature Name Example Species ADHERENCE/COLONIZATION FACTORSRickettsia 56 kDa type-specific antigen protein Rickettsia tsutsugamushiAdhesin B Streptococcus pneumoniae Neisseria meningitidis adhesin MafBNeisseria meningitidis Adhesin family Streptococcus pneumoniaeEscherichia coli P pili tip fibrillum papE protein Escherichia coliEscherichia coli P pili tip fibrillum papF protein Escherichia coliPorphyromonas gingivalis fimbrillin protein Porphyromonas gingivalisEscherichia coli P pili regulatory PapB protein Escherichia coliFlagellin Salmonella spp. Flagellar biosynthetic protein FliP Salmonellaspp. Flagellar protein FlgJ Salmonella spp. Flagellar assembly proteinFliH Salmonella spp. Flagellar FliJ protein Salmonella spp. Flagellarhook-associated protein Salmonella spp. Flagellar hook-basal bodycomplex protein FliE Salmonella spp. Flagellar hook-length controlprotein Salmonella spp. Flagellar L-ring protein Salmonella spp.Flagellar motor switch protein FliG Salmonella spp. Flagellar motorswitch protein FliM Salmonella spp. Flagellar motor switch protein FliNSalmonella spp. Flagellar M-ring protein Salmonella spp. FlagellarP-ring protein Salmonella spp. Macrophage infectivity potentiatorLegionella pneumophila Sodium-type flagellar protein MotY precursorSalmonella spp. Yersinia outer membrane adhesin Yersinia spp. Type IVprepilin cysteine protease (C20) family Escherichia coli Lactobacillussurface layer protein Lactobacillus brevis INVASINS GalliderminStreptococcus mutans IgA-specific serine endopeptidase (S6) Haemophilusinfluenzae Microbial collagenase metalloprotease (M9) Clostridiumperfringens Bacterial phospholipase A1 protein Escherichia coliProkaryotic zinc-dependent phospholipase C Clostridium perfringensStreptothricin acetyltransferase Escherichia coli Streptococcusstreptokinase protein Streptococcus spp. Thermolysin metalloprotease(M4) family Bacillus spp. Urea amidohydrolase (urease) proteinEscherichia coli CELL SURFACE FACTORS 60 kDa inner membrane proteinEscherichia coli Yersinia/Haemophilus virulence surface antigen Yersiniapestis Bacterial lipocalin Vibrio cholerae Nucleoside-specificchannel-forming protein Tsx Escherichia coli Chlamydia cysteine-richouter membrane protein 3 Chlamydia trachomatis Chlamydia cysteine-richouter membrane protein 6 Chlamydia trachomatis Chlamydia major outermembrane protein Chlamydia trachomatis E. coli/Salmonella-type porinEscherichia coli E. coli/Neisseria porin superfamily Escherichia coliEnterobacterial virulence outer membrane protein Escherichia coliGram-positive coccus surface protein anchor Streptococcus pyogenes Icenucleation protein Pseudomonas spp. Low calcium response V antigenYersinia pestis LPP20 lipoprotein precursor Helicobacter pyloriNeisseria sp. porin Neisseria spp. Nucleotide sugar epimerase Vibriovulnificus OMPA domain Escherichia coli OMS28 porin Borrelia burgdoferiOuter membrane protein A Escherichia coli Outer surface protein Borreliaburgdoferi EXOTOXINS Aerolysin Aeromonas hydrophila Anthrax toxin LFsubunit Bacillus anthracis Salmonella/Shigella invasin protein CSalmonella spp. Salmonella/Shigella invasin protein B Salmonella spp.Bacterial toxin Streptococcus pyogenes Staphylococcal bi-component toxinStaphylococcus aureus Clostridial binary toxin A Clostridium spp. Binarytoxin B family Clostridium spp. Bontoxilysin Clostridium botulinumBordetella pertussis toxin A subunit Bordetella pertussis Bordetellapertussis toxin B subunit Bordetella pertussis NodO calcium bindingEscherichia coli Cytolethal distending toxin A Escherichia coliCytolethal distending toxin B Escherichia coli Channel forming colicinEscherichia coli Clostridium enterotoxin Clostridium spp. CloacinEscherichia coli Diphtheria toxin Corynebacterium dihtheriae Heat-labileenterotoxin A chain Escherichia coli Heat labile enterotoxin B chainEscherichia coli Fragilysin metallopeptidase (M10C) enterotoxinBacteroides fragilis Helicobacter neutrophil-activating protein A familyHelicobacter pylori Intimin Escherchia coli Salmonella/Shigella invasionprotein E (InvE) Salmonella spp. Mycoplasma P48 major surfacelipoprotein Mycoplasma fermentans Nisin Bacillus subtilis Omptin serineprotease Escherchia coli Pertactin Bordetella pertussis Pertactinvirulence factor family Bordetella pertussis Pyocin S killer proteinPseudomonas aeruginosa Ribosome inactivating protein family Escherichiacoli Salmonella type III secretion SopE effector protein Salmonella spp.Streptomyces subtilisin inhibitor Streptomyces coelicolor Invasionprotein B family Salmonella spp. Salmonella invasion protein InvJSalmonella spp. Streptopain (C10) cysteine protease family Streptococcuspyogenes Subtilisin serine protease family (S8) Bacillus spp. Bacterialthiol-activated pore-forming cytolysin Streptococcus pyogenesStaphylococcal toxic shock syndrome toxin Staphylococcus aureusTranslocated intimin receptor (Tir) Escherichia coli Type III secretionsystem outer membrane B protein family Shigella spp. Type IV secretionsystem CagA exotoxin Helicobacter pylori V8 serine protease familyStaphylococcus aureus Helicobacter pylori vacuolating cytotoxinHelicobacter pylori Yersinia virulence determinant YopE protein Yersiniapestis Yersinia serine/threonine protein kinase Yersinia pestisTRANSPORTERS ABC-2 type transport system membrane protein Haemophilusinfluenzae Salmonella/Yersinia modular tyrosine phosphatase Salmonellaspp. Bacterial general secretion pathway protein C Vibrio choleraeBacterial general secretion pathway protein D Vibrio cholerae Bacterialgeneral secretion pathway protein F Vibrio cholerae Bacterial generalsecretion pathway protein G Vibrio cholerae Bacterial general secretionpathway protein H Vibrio cholerae Gram-negative bacterial RTX toxindeterminant A family Escherichia coli Gram-negative bacterial RTXtoxin-activating protein C Escherichia coli Gram-negative bacterial RTXsecretion protein D Escherichia coli SecA protein Escherichia coliBacterial protein-transport SecB chaperone protein Escherichia coliBacterial translocase SecE Escherichia coli Bacterial translocase SecFprotein Escherichia coli Protein-export SecG membrane proteinEscherichia coli Preprotein translocase SecY subunit Escherichia coliSalmonella surface presentation of antigen gene type M Salmonella spp.Gram-negative bacterial type III secretion SycD chaperone Yersiniapestis Gram-negative bacterial type III secretion SycE chaperoneYersinia pestis Bacterial sec-independent translocation TatB proteinEscherichia coli Gram-negative bacterial tonB protein Escherichia coliType III secretion system inner membrane A protein family Salmonellaspp. Type III secretion system inner membrane P protein familySalmonella spp. Type III secretion system inner membrane Q proteinfamily Salmonella spp. Type III secretion system inner membrane Rprotein family Salmonella spp. Type III secretion system inner membraneS protein family Salmonella spp. Type III secretion system outermembrane G protein family Salmonella spp. Type III secretion systemouter membrane K protein family Salmonella spp. Type III secretionsystem outer membrane O protein family Salmonella spp. Type IV secretionsystem CagX conjugation protein Helicobacter pylori SIDEROPHORES Ferriciron reductase Escherichia coli 2,3-dihydro-2,3-dihydroxybenzoatedehydrogenase Escherichia coli Enterobactin synthetase component DEscherichia coli Ferrichrome-binding periplasmic Escherichia coliIsochorismatase Escherichia coli MISCELLANEOUS Acetate kinase familyEscherichia coli Acriflavin resistance protein family Escherichia coliAlanine racemase Escherichia coli Bacterial arginine deiminaseClostridium perfringens Bacterial arginine represser Bacillus spp.Autoinducer synthesis protein Yersinia spp. Beta-lactamase class AEscherichia coli Bacterial carbamate kinase Pseudomonas aeruginosaDNA-binding protein FIS Escherichia coli Prokaryotic integration hostfactor Streptococcus spp. Hok/Gef cell toxic protein family Escherichiacoli FIS bacterial regulatory protein HTH Escherichia coli TetRbacterial regulatory protein HTH Escherichia coli Lipopolysaccharidecore biosynthesis protein Escherichia coli Bacterial autoinducer-2(AI-2) production protein LuxS Escherichia coli Maltose binding proteinEscherichia coli Bacterial mechano-sensitive ion channel Escherichiacoli Metalloprotease inhibitor Pseudomonas spp. Hypothetical mycoplasmalipoprotein (MG045) Mycoplasma app. Gene IV protein Vibrio choleraePhosphoenolpyruvate-protein phosphotransferase Legionella pneumophilaSalmonella virulence plasmid 28.1 kDa A protein Salmonella spp.Salmonella virulence plasmid 65 kDa B protein Salmonella spp. Bacterialcell shape determinant MreB/Mbl protein Bacillus subtilis Tetracyclineresistance protein Escherichia coli Tetracycline resistance protein TetBStreptococcus spp. Tetracycline resistance protein TetO/TetQ/TetM familyStreptococcus spp. Tetracycline represser protein Escherichia coliNON-BACTERIAL VIRULENCE FACTORS Alpha giardin Giardia lamblia Densegranule Gra2 protein Toxoplasma gondii Dense granule Gra6 proteinToxoplasma gondii Dense granule Gra7 protein Toxoplasma gondii MIC1microneme protein Toxoplasma gondii

1. A method of detecting a cell expressing a target gene comprising thesteps of: a. introducing to said cell a nucleic acid construct encodinga portion of said target gene sequence, fused to an effector genesequence, and a construct encoding an inhibitory nucleic acid sequencedirected against at least said portion of said target gene sequence,wherein in the absence of said target gene, said inhibitory nucleic acidsequence suppresses effector gene expression; and b. monitoringexpression of said effector gene sequence in said cell and comparing thelevel of effector gene expression to a reference level, wherein ifexpression of said effector gene is increased relative to said referencelevel, said target gene is detected in said cell.
 2. The method of claim1, wherein the target gene is a mutant gene.
 3. The method of claim 1,wherein said nucleic acid construct encoding a portion of said targetgene sequence, fused to a said effector gene sequence, and saidconstruct encoding an inhibitory nucleic acid sequence are encoded onthe same nucleic acid molecule.
 4. The method of claim 1, wherein thecell is a bacterial cell.
 5. The method of claim 1, wherein the cell isa eukaryotic cell.
 6. The method of claim 1, wherein the target gene isa drug resistance or virulence gene.
 7. The method of claim 1, whereinthe target gene is a multi-drug resistance (MDR) transporter gene. 8.The method of claim 1, wherein the target gene is a viral gene.
 9. Themethod of claim 1, wherein the target gene is an oncogene or tumorassociated gene.
 10. The method of claim 1, wherein the effector genesequence encodes a reporter molecule.
 11. The method of claim 1, whereinthe reporter comprises a polypeptide selected from the group consistingof an enzyme, a fluorescent polypeptide, a luminescent polypeptide andan antigen.
 12. An engineered gene sensor comprising a nucleic acidsequence encoding a portion of a target gene sequence fused to aneffector gene sequence and further comprising a construct encoding aninhibitory nucleic acid sequence directed against at least said portionof said target gene sequence, wherein the presence of the target geneexpression in a cell permits said effector gene expression to produce aneffector polypeptide which mediates delivery of an agent or mediatescell killing.
 13. The gene sensor of claim 12, wherein said nucleic acidconstruct encoding a portion of said target gene sequence, fused to asaid effector gene sequence, and said construct encoding an inhibitorynucleic acid sequence are encoded on the same nucleic acid molecule. 14.The gene sensor of claim 12, wherein said nucleic acid moleculecomprises a vector. 15-20. (canceled)
 21. A method of selectivelykilling a cell expressing a target gene, the method comprising the stepsof introducing to said cell a nucleic acid construct encoding a portionof said target gene sequence, fused to an effector gene sequence, and aconstruct encoding an inhibitory nucleic acid sequence directed againstat least said portion of said target gene sequence, wherein in theabsence of target gene expression, said inhibitory nucleic acid sequencesuppresses expression of said effector gene, and wherein the presence oftarget gene expression in said cell permits effector gene expression toproduce an effector polypeptide, and wherein expression of said effectorpolypeptide mediates cell killing. 22-45. (canceled)
 46. A kit fordetecting a target gene, the system comprising: c. a nucleic acidconstruct encoding a portion of said target gene sequence, fused to aneffector gene sequence; d. a construct encoding an inhibitory nucleicacid sequence that inhibits said target gene expression, wherein thenucleic acid constructs of (a) and (b) are comprised by the same nucleicacid molecule; and e. packaging materials therefor.
 47. The kit of claim46, wherein said nucleic acid construct encoding a portion of saidtarget gene sequence, fused to a said effector gene sequence, and saidconstruct encoding an inhibitory nucleic acid sequence are encoded onthe same nucleic acid molecule.
 48. The kit of claim 46, wherein saidnucleic acid molecule comprises a vector. 49-56. (canceled)